OPINION OF THE COURT
NYGAARD, Circuit Judge.Appellants, Charles Radich and Howard Walton, appeal from an order granting summary judgment against them in an action for damages under 42 U.S.C. § 1983 and 42 U.S.C. § 1985 which alleged that they were arrested without probable cause in violation of their constitutional rights. Appellants essentially raise two issues: whether the district court granted summary judgment prematurely; and, whether the police officer had probable cause to arrest appellants for defiant trespass. Because we conclude that the record shows no dispute of any material facts, that appellants offer no valid reason why summary judgment should have been deferred, and that the arrests were upon probable cause, we will affirm.
I.
On January 23, 1987, Officer Mieklejohn arrested appellants for defiant trespass. Appellants, pro-life supporters who had been protesting on a public sidewalk, crossed onto a private parking lot to distribute pro-life literature and speak to individuals entering the Northeast Women’s Center, Inc. (the Center), a clinic that performs abortions. The parking lot in question, which was delineated by a white line painted on the ground, was posted with a sign which stated that the lot was private property and that the owners denied permission for protesters to come onto the lot. Additionally, Officer Mieklejohn warned appellants that they would be arrested if they crossed onto the parking lot.
Appellant Radich first crossed the line and entered onto the parking lot as he went to his car located in a different parking lot. While he was returning, he approached within fifteen feet of the Center’s entrance. At this point, Officer Mieklejohn, because protestors had invaded the Center in the past and because the parking lot owners did not want protestors on the parking lot, warned Radich that he would be arrested if he crossed onto the parking lot again. Within a short period of time, Radich crossed onto the parking lot and approached the clinic entrance. Officer Miek-lejohn then arrested Radich for defiant trespass.
When Radich was arrested, appellant Walton, who had also been warned earlier to stay out, crossed the line and entered the lot and demanded to know why Radich was being arrested. Several times Officer Mieklejohn told Walton not to interfere with the arrest and to leave the property. After Walton refused to leave, the officer arrested him for defiant trespass. The charge against each appellant was dropped for lack of prosecution.
Before the arrests, Comly Road Associates, owners of the parking lot, told the city that they were not granting anyone permission to protest on their property. In turn, the City Solicitor’s office advised the police department that “the subject property was private, and that since protestors had not been given permission to conduct protest activity thereon, the Department had the right to take appropriate action within their judgment to protect that private property.” Affidavit of John Meyers. Then, Officer Mieklejohn’s superiors in the police department told him that the parking lot was private property and that the owners had denied the protestors permission to *1393protest on the lot. Affidavit of Harry Mieklejohn.
Appellants filed their complaint on November 17, 1987 alleging that appellees violated appellants’ first, fourth, fifth, eight and fourteenth amendment rights, as well as a variety of pendent state claims. On February 17, 1988, the district court ordered discovery to close on March 22, 1988 and set trial for March 24, 1988. On February 22, 1988, appellees moved for summary judgment; on March 9, 1988, appellants responded. Apparently, the case was then suspended from March 30, 1988 until June 15, 1988 when the court issued an order reactivating it. Brief for Appellees at 3. On June 30, 1988, the district court extended the period for discovery until August 31, 1988 and set the trial for the first week of September. On July 6, 1988, both parties submitted supplemental memoranda on the summary judgment motion. On September 6, 1988, the district court granted appellees’ motion for summary judgment and dismissed all pendent state claims without prejudice.
The district court granted summary judgment for all defendants on appellants’ Section 1983 claim because it determined that Officer Mieklejohn had probable cause to arrest appellants.1 The district court relied upon four facts to establish that Officer Mieklejohn had probable cause to arrest appellants for defiant trespass: (a) the sign at the border of the parking lot stated that protestors were not allowed to protest in the parking lot; (b) a line was painted which indicated where the private property began; (c) Officer Mieklejohn previously warned appellants that they were on private property and told them to leave or be arrested; and (d) appellants, in paragraphs 13 and 14 of their complaint, admitted that they were in the parking lot to pursue their protest activities.
II.
Appellants first argue that the district court committed reversible error in granting summary judgment before appellants were able to obtain answers to their interrogatories. Whether a district court prematurely grants summary judgment is reviewed for abuse of discretion. Dowling v. City of Philadelphia, 855 F.2d 136 (3d Cir.1988).
The court must give a party opposing summary judgment an adequate opportunity to obtain discovery. Dowling, 855 F.2d at 139. Rule 56(f) of the Federal Rules of Civil Procedure gives the district court discretion to defer ruling on a summary judgment motion when a party opposing summary judgment files an affidavit indicating that it needs more discovery. Rule 56(f) provides,
(f) When Affidavits are Unavailable. Should it appear from the affidavits of a party opposing the motion that the party cannot for reasons stated present by affidavit facts essential to justify the party’s opposition, the court may refuse the application for judgment or may order a continuance to permit affidavits to be obtained or depositions to be taken or discovery to be had or may make such other order as is just.
This circuit generally requires that a party file a Rule 56(f) affidavit in order to preserve the issue for appeal. Galgay v. Gil-Pre Corp., 864 F.2d 1018 (3d Cir.1988); Dowling, 855 F.2d 136; Wisniewski v. Johns-Manville Corp., 812 F.2d 81 (3d Cir.1987); Falcone v. Columbia Pictures Industries, Inc., 805 F.2d 115 (3d Cir.1986); Mid-South Grizzlies v. National Football League, 720 F.2d 772 (3d Cir.1983), cert. denied, 467 U.S. 1215, 104 S.Ct. 2657, 81 L.Ed.2d 364 (1984); but see, Sames v. Gable, 732 F.2d 49, 52 n. 3 (3d Cir.1984) (court held it was error for district court to grant summary judgment when it knew discovery was still pending even though plaintiffs did not comply with Rule 56(f)). Beyond the procedural requirement of filing an affidavit, Rule 56(f) also requires that a party indicate to the district court its need for discovery, what material facts it hopes to *1394uncover and why it has not previously discovered the information. Hancock Industries v. Schaeffer, 811 F.2d 225 (3d Cir. 1987); Koplove v. Ford Motor Co., 795 F.2d 15 (3d Cir.1986).
Here, appellants did not file a Rule 56(f) affidavit. Appellants contend that their attorney’s unverified memorandum opposing the motion for summary judgment complies with the Rule 56(f) affidavit requirement. It does not. Rule 56(f) clearly requires that an affidavit be filed. “The purpose of the affidavit is to ensure that the nonmoving party is invoking the protection of Rule 56(f) in good faith and to afford the trial court the showing necessary to assess the merit of a party’s opposition.” First Chicago International v. United Exchange Co., LTD, 836 F.2d 1375 (D.C.Cir.1988) (citing First National Bank of Arizona v. Cities Service Co., 391 U.S. 253, 298, 88 S.Ct. 1575, 1597, 20 L.Ed.2d 569 (1968)). An unsworn memorandum opposing a party’s motion for summary judgment is not an affidavit.
Appellants further argue that they essentially have complied with the remaining requirements of Rule 56(f) and thus we should overlook their failure to file an affidavit and reverse the district court’s grant of summary judgment. The argument will not withstand close scrutiny.
Appellees filed their motion for summary judgment on February 22, 1988. Appellants answered the motion on March 9, 1988. In their unverified memorandum opposing the summary judgment motion they make the following contentions:
Municipal Defendants’ Motion, however, in addition to being inappropriate on its merits, is premature. Plaintiffs have filed a set of interrogatories against Municipal Defendants which must be answered ... evidence to be presented on behalf of plaintiffs is incomplete.
[A]ll of the facts are not in yet with reference to the policy underlying the decision of the police department to arrest plaintiffs. What is clear even from the affidavits attached to Municipal Defendants’ Motion is that such a policy existed. It is argued by plaintiffs in consideration of the affidavit of Charles Volz, Esquire, attached hereto and made part hereof as Exhibit “B” that a clear and direct policy of Municipal Defendants to abort the constitutional guarantees of plaintiffs was in existence and was continually enforced and to this day is being enforced by the Municipal Defendants ... The evidence to be presented by plaintiffs would establish that they were arrested in the parking lot of this shopping center merely because they were known by police to be individuals involved in “pro life” activities.
Appendix at 131a-134a.
Appellants’ claim of outstanding interrogatories may have been true in March, however, summary judgment was granted in September, and, in the interim, appellants failed to explain whether the interrogatories were still outstanding or why they had been unable to get the interrogatories answered, if that was the case. In fact, in mid-June, 1988, the district court requested the parties to update their research on their respective summary judgment motions. In late June, the district court extended the deadline for discovery until August 31, 1988. On July 6, 1988, appellants responded with their supplemental brief. Nowhere in this supplemental brief do appellants reassert any need for discovery, even though four months had elapsed since they first claimed that interrogatories were unanswered. Further, they did not file a motion to compel answers to their interrogatories; nor were there any other motions by either party which would alert the district court that the interrogatories remained unanswered.
Additionally, the statements made by appellants in their memorandum submitted in March state neither the content of the unanswered interrogatories nor “what particular information is sought, [and] how, if uncovered, it would preclude summary judgment.” Dowling, 855 F.2d at 140. Furthermore, the statements in the memorandum were not made by a witness or other affiant, but in the course of advocacy by appellant’s attorney. We cannot dimin*1395ish the value of an affidavit by permitting an attorney’s unsworn statement to replace it. The adversary system recognizes the right and practice of attorneys to take adversarial license with evidence and argue it as fact. It does not recognize argument as a surrogate for either evidence or fact. Thus, the statement is lacking both in substance, and in any indicia of evidentiary reliability contemplated by the requirements of Rule 56.
In light of appellants’ failure to comply with Rule 56(f), we conclude that the district court did not abuse its discretion in granting summary judgment when it did.
III.
Appellants next argue that the district court erred in granting summary judgment for appellees based on its conclusion that the officers had probable cause to arrest appellants. Our standard of review is plenary. Dowling, 855 F.2d 136.
Summary judgment is proper “if the pleadings, depositions, answers to interrogatories, and admissions on file, together with the affidavits, if any, show that there is no genuine issue as to any material fact and that the moving party is entitled to judgment as a matter of law.” Fed.R.Civ.P. 56(c). The moving party is entitled to a judgment as a matter of law if the non-moving party fails to establish the existence of a genuine issue on an essential element of her case on which she has the burden of proof at trial. Celotex Corp. v. Catrett, 477 U.S. 317, 106 S.Ct. 2548, 91 L.Ed.2d 265 (1986). A genuine issue is not made unless the evidence, viewed in a light most favorable to the nonmoving party, would allow a reasonable jury to return a verdict for that party. Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 248-49, 106 S.Ct. 2505, 2510-11, 91 L.Ed.2d 202 (1986). If the evidence is merely colorable or is not significantly probative, summary judgment may be granted. Id. at 249-50, 106 S.Ct. at 2510-11. Whether a fact is material is determined by substantive law. Id. at 248, 106 S.Ct. at 2510.
To succeed in a Section 1983 action, a claimant must prove that a person acting under color of state law violated the claimant’s constitutional rights. Robb v. City of Philadelphia, 733 F.2d 286 (3rd Cir.1984). Appellants claim that the City of Philadelphia and Officer Mieklejohn violated appellants’ constitutional rights because Officer Mieklejohn arrested them pursuant to a policy of the City to arrest “pro-lifers,” and not upon probable cause. Although an arrest without probable cause is a constitutional violation actionable under Section 1983, Patzig v. O’Neil, 577 F.2d 841, 848 (3rd Cir.1978), liability can be imposed on a city only when the claimant proves that the unconstitutional arrest was caused affirmatively by a policy of that city,2 City of Oklahoma City v. Tuttle, 471 U.S. 808, 823, 105 S.Ct. 2427, 2436, 85 L.Ed.2d 791 (1985); and, liability can be imposed on a police officer only if “on an objective basis, it is obvious that no reasonably competent officer would have concluded that” probable cause existed. Malley v. Briggs, 475 U.S. 335, 341, 106 S.Ct. 1092, 1096, 89 L.Ed.2d 271 (1986).
The district court granted summary judgment in favor of the city and Officer Mieklejohn because Officer Mieklejohn had probable cause to arrest appellants. The issue before us then is whether, viewing the evidence in a light most favorable to appellants, Officer Mieklejohn had probable cause to arrest appellants, or alternatively, whether appellants suffered a constitutional injury.
Probable cause exists when the facts and circumstances are “sufficient to warrant a prudent man in believing that the [suspect] *1396had committed or was committing an offense.” Beck v. Ohio, 379 U.S. 89, 91, 85 S.Ct. 223, 225, 13 L.Ed.2d 142 (1964), quoted in, Gerstein v. Pugh, 420 U.S. 103, 111, 95 S.Ct. 854, 861, 43 L.Ed.2d 54 (1975).3 Thus, we must determine whether the facts and circumstances would lead a reasonable officer to believe that the offense of defiant trespass had been committed.
The Pennsylvania Defiant Trespass Statute provides, in pertinent part,
(b) Defiant trespasser.—
(1) A person commits an offense if, knowing he is not licensed or privileged to do so, he enters or remains in any place as to which notice against trespass is given by:
(1) actual communication to the actor; or
(ii) posting in a manner prescribed by law or reasonably likely to come to the attention of intruders; or
(iii) fencing or other enclosure manifestly designed to exclude intruders.
(c) Defenses.—It is a defense to prosecution under this section that:
(2) the premises were at the time open to members of the public and the actor complied with all lawful conditions imposed on access to or remaining in the premises;
18 Pa.Cons.Stat.Ann. § 3503.
Appellants do not refute that their actions constitute the offense of defiant trespass. They claim, however, that the premises were open to the public and the condition imposed by the parking lot owners is unlawful and consequently the officer had no probable cause to arrest them. Appellants argue that the district court erred when it found probable cause without considering whether they had an affirmative defense. We disagree. The facts and circumstances support the district court’s conclusion.
The Pennsylvania Courts have classified Section 3503(c) as an affirmative defense. Commonwealth v. Downing, 511 Pa. 28, 511 A.2d 792, 793, 795 (1986); Commonwealth v. White, 342 Pa.Super. 1, 492 A.2d 32, 35 (1985). In Pennsylvania, “an affirmative defense is defined as one where the defendant admits his commission of the act charged, but seeks to justify or excuse it.” White, 492 A.2d at 35. See, also, Commonwealth v. Rose, 457 Pa. 380, 387, 321 A.2d 880 (1974). Although the burden of persuasion falls on the Commonwealth to disprove the affirmative defense, the defendant must introduce some evidence in support of the defense. White, 492 A.2d at 35-36. Thus, the onus is on the appellants to come forward with facts that support the affirmative defense. White, 492 A.2d at 35. Here, appellants must not only show facts which support the defense, but also that a reasonable police officer would know of these facts and conclude that a defiant trespass had not been committed.
To establish the affirmative defense of Section 3503(c)(2), a defendant must introduce evidence showing both that the premises were open to the public at the time of entry and that he complied with all lawful conditions imposed upon access. Accordingly, assuming arguendo that appellants are correct that to establish probable cause for arrest under Pennsylvania’s defiant trespass statute the arresting officer must consider the statute’s affirmative defense, to prove that probable cause was lacking, appellants must demonstrate that the arresting police officer, acting reasonably, would have believed: (1) that the property was “open to the public,” and (2) that the appellants were complying with lawful conditions, or that the conditions imposed upon access were unlawful. Here, appellants may be able to show that the parking lot was open to segments of the public.4 Appellants cannot show, however, that they complied with the condition imposed by the parking lot owners that protesting was for*1397bidden on the parking lot.5 Consequently, the success of appellants’ claim that they were arrested without probable cause, hinges on whether a reasonable police officer, under the facts and circumstances of this case, would know that the condition was unlawful and therefore, that appellants had an affirmative defense to the defiant trespass statute.
Whether the condition imposed on access to the parking lot under these circumstances is lawful is a question of law not readily answered by precedents applying Section 3503. It is, however, a question we need not answer here; as we have noted, the standard for probable cause turns not on the actual guilt or innocence of the arres-tee, but rather, whether the arresting officer reasonably believed that the arrestee had committed the crime. To support their claim that the police acted unreasonably, appellants argue that because the parking lot was “open to the public” they had an “unfettered right” under Section 3503 to engage in expressive activity there. Therefore, appellants urge, the prudent police officer would have known that the parking lot owner’s prohibition on demonstrations in the parking lot was illegal.6 Contrary to the appellant’s assertions, the Pennsylvania Courts have not clearly interpreted Section 3503 to assure protestors of an “unfettered right” to engage in expressive activity on private property that is “open to the public,” as that term appears in Section 3503. In Commonwealth v. Tate, 432 A.2d 1382 (1981), the Pennsylvania Supreme Court held that under the Pennsylvania Constitution, a private college could not arbitrarily prohibit persons from distributing political literature on the campus. Therefore, the court held, the affirmative defense to the criminal trespass statute applied to those persons distributing the literature, and they could not be convicted under Section 3503.
The Tate holding rested heavily on the notion that although the university was private, it had created a privately owned forum open to the community at large for expressive activities. The court stated,
Here we are faced with an educational institution which holds itself out to the public as a community resource and cultural center, allows members of the public to walk its campus, permits a community organization to use its facilities as a forum for a public official of national importance, and at the same time arbitrarily denies a few members of the public the right to distribute leaflets relevant to that forum.
Id. at 1387. Accordingly, the Tate court drew support for its holding from the California and New Jersey decisions which likewise rested on the premise that the property at issue was a forum either created for or compatible with expressive activity. See id. at 1389 (citing Robins v. PruneYard Shopping Center, 23 Cal.3d 899, 908, 592 P.2d 341, 153 Cal Rptr. 854, 859 (1979) (state constitution required large shopping center to permit expressive activity on premises) and State v. Schmid, 84 N.J. 535, 423 A.2d 615 (1980) (state constitution requires that Princeton University allow distribution of political material on campus)). Additionally, the Tate court held that it was not relevant that the protestors had distributed the materials without a permit, as the college’s standardless permit requirement was unconstitutional as a matter of state constitutional law. The Tate decision did not declare, however, that on every sort of private property “open to the pub-*1398lie,” the citizens have an unfettered right to engage in expressive activities.
Thereafter, in Western Pennsylvania Socialist Workers 1982 Campaign v. Connecticut Gen. Life Ins. Co., 512 Pa. 23, 515 A.2d 1331 (1986) (plurality opinion), the Pennsylvania Supreme Court held that privately owned shopping malls may refuse to permit citizens to solicit on premises signatures for political purposes. In so holding, the plurality distinguished a shopping mall owner who “uniformly precludes political activity on his property” and the “public forum” provided by the college in Tate. Western Pennsylvania, 515 A.2d at 1336— 37. The plurality noted that unlike the college, the privately owned shopping mall "operated as a market place for the exchange of goods and services ... not as a market place for the exchange of ideas.” Id. at 1337. As the case involved a private injunction rather than criminal prosecution for trespass, two of the judges on the panel of six also suggested that the result might be different if the state were prosecuting a charge of defiant trespass under the same circumstances. See id. at 1337 n. 6 (plurality opinion) and at 1340 (Zappala, J., concurring).
This brief survey of Pennsylvania law illustrates that the legality of the condition imposed here is not readily discernable to a police officer making arrests or city official formulating a policy to enforce private trespass rights. We emphasize that appellants were arrested on a parking lot, dedicated not to expressive activity, but to the temporary storage of motor vehicles. A parking lot used only for commercial purposes bears no resemblance to a college campus that has opened its doors to general debate, or even to a shopping mall, where pedestrians assemble and stroll at leisure. Appellants have introduced no evidence to suggest that the parking lot owners have allowed other sorts of protesting activity on the lot or have otherwise operated the property as “a market place for the exchange of ideas.” Therefore, under these circumstances, we cannot impose upon a police officer or a city official authorizing the arrest policy the duty to correctly predict how a court will answer this unresolved and complex legal issue. Cf. Michigan v. De Fillippo, 443 U.S. 31, 38-39, 99 S.Ct. 2627, 2632-33, 61 L.Ed.2d 343 (1979) (probable cause to arrest existed even where the ordinance was later found unconstitutional because “[a] prudent police officer ... should not have been required to anticipate that a court would later hold the ordinance unconstitutional”).
Appellants next argue that their arrests violated their first amendment right to freedom of speech. The First and Fourteenth Amendments of the United States Constitution prevent a state from “abridging the freedom of speech.” U.S. Const. Amends. I, XIV. The first and fourteenth amendments, however, do not prevent an individual from restricting the exercise of free speech on private property. Hudgens v. NLRB, 424 U.S. 507, 96 S.Ct. 1029, 47 L.Ed.2d 196 (1976). Private property has been treated as public property for first amendment purposes only when such property has all of the characteristics of a town. See, Marsh v. Alabama, 326 U.S. 501, 66 S.Ct. 276, 90 L.Ed. 265 (1946). In Lloyd Corp. v. Tanner, 407 U.S. 551, 569, 92 S.Ct. 2219, 2229, 33 L.Ed.2d 131 the court further explained that private property does not “lose its private character merely because the public is generally invited to use it for designated purposes.” In this case, the premises are designated as a parking lot. It has not lost its private character and appellants may not invoke the protection of the First and Fourteenth Amendments of the United States Constitution.
Appellants’ next argue that using the police officer to arrest appellants for exercising their first amendment rights constitutes “state action.” This extends the concept of state action too far. Police action to protect private property rights does not thereby subject that private property to first amendment constraints. In a similar situation, the First Circuit has held,
Plaintiffs are attempting to create a first amendment right of access simply from the police involvement in arresting them. This bootstrap argument would turn any arrest in support of private rights into *1399state action, thereby eviscerating the requirement.
Cape Cod Nursing Home v. Rambling Rose Rest Home, 667 F.2d 238, 243 (1st Cir.1981). We agree with the First Circuit’s analysis and reject appellants’ argument.
In summary, appellants were arrested because they failed to heed both the posted signs and the verbal warnings of the arresting officer, who had probable cause to believe a violation of the defiant trespass had occurred. We will affirm.
. The district court also granted summary judgment on the Section 1985 claim, but appellants have not pursued an appeal on that claim.
. The record reveals some evidence of what could arguably be considered a "city policy" with respect to the arrests of appellants. See e.g., Affidavit of John M. Myers (city solicitor’s office intended to advise police department that it "had the right to take appropriate action within their judgment to protect that private property”). However, since we conclude that no constitutional violation occurred because Officer Mieklejohn had probable cause to arrest appellants, the city cannot be liable. Los Angeles v. Heller, 475 U.S. 796, 106 S.Ct. 1571, 89 L.Ed.2d 806 (1986).
. The federal standard for probable cause is the proper standard to apply. Fisher v. Washington Metro. Area Transit Authority, 690 F.2d 1133, 1138 (4th Cir.1982).
. There is evidence in the record that people other than the business customers of the adjacent business cross into the parking lot.
. Appellants concede in their brief that the lot was open to anti-abortion advocates who did not wish to distribute anti-abortion literature.
. Appellants also argue that "mere presence” on private property is not enough to support a conviction for criminal trespass under Pennsylvania law, citing White for the proposition that only conduct that constitutes a "breach of the peace" qualifies as defiant trespass. Appellants misread White. In that case, the court found that defendant had complied with all lawful conditions imposed on access, as he "was present [on the property] ... for the explicit purpose for which the area was open.” White, 492 A.2d at 35. In conclusion, the court stated, "the Commonwealth presented no evidence that defendant had failed to comply with any lawful condition on his entry to access or in any other way 'breached the peace.' ” Id. at 36 (emphasis added).