After a jury trial in the Northampton Division of the District Court Department, the defendant was convicted under G. L. c. 269, § 2, of failing to obey the order of a police officer to disperse from an “unlawful assembly.” On appeal, the defendant argues that the phrase “riotously or tumultuously assembled,” set forth in the statute, should be read conjunctively with the term “unlawful assembly” to define the offense, and that when the judge omitted this phrase from his instructions, the jury were precluded from considering an essential element of the offense. In addition, the defendant contends that the statute is unconstitutional on its face, claiming that it is vague *577and overbroad. He also argues that the judge erred in failing to give an instruction that, under the First Amendment to the United States Constitution and arts. 16 and 19 of the Massachusetts Declaration of Rights, the people have the right to assemble to engage in constitutionally protected speech and petition the government.
The trial transcript has not been made part of the record on appeal.1 Without a complete record we will not reach either the question whether the statute was unconstitutional as applied, or claimed errors in the judge’s instructions to the jury. We consider only the defendant’s claim that the statute is unconstitutional on its face and the question whether the phrase “riotously or tumultuously assembled” should be read conjunctively to describe the offense of unlawful assembly.
Although, in the absence of a transcript, we have no information as to what actually transpired, we discern from other materials included in the appellate record that what occurred, generally, was as follows. The defendant, along with other persons, participated in an assembly in Northampton to protest the involvement of the United States in the Iraq war. At least some persons involved in the protest may have been on a sidewalk. The defendant was subsequently arrested for refusing to disperse upon the command of a police officer.2
Defendant’s arguments on interpretation of statute. The defendant argues that on its face the statute is unconstitutionally overbroad and vague unless the offense of unlawful assembly is qualified by the additional statutory terms “riotous or tumultuous.”
The offense of refusing or failing to obey an order to disperse from an unlawful assembly is set forth in two sections of G. L. c. 269. The conduct at which the statute is directed is set out in G. L. c. 269, § 1, as amended through St. 1991, c. 412, § 98,3 the relevant portion of which reads as follows:
*578“If five or more persons, being armed with clubs or other dangerous weapons, or if ten or more persons, whether armed or not, are unlawfully, riotously or tumultuously assembled in a city or town, . . . any member of the city, town, or state police . . . shall go among the persons . so assembled . . . and . . . command all persons so assembled immediately and peaceably to disperse . . . .”
In the event those ordered to disperse fail to do so, the following portion of G. L. c. 269, § 2, as appearing in St. 1965, c. 647, § 1A, sets forth the authority to arrest and punish:
“Whoever ... if required by such . . . officer to depart from the place, refuses or neglects so to do, shall be considered one of the rioters or persons unlawfully assembled, and shall be [subject to criminal prosecution and punishment].”
We think the defendant’s argument that “unlawful assembly” must be qualified by “riotous or tumultuous” is flawed in two respects. First, his argument is based on decisional law interpreting a related, but separate offense, set out by G. L. c. 269, § 8, which requires a municipality to pay damages caused by a riotous or tumultuous assembly that it had an obligation to control. See Yalenezian v. Boston, 238 Mass. 538, 542-543 (1921); Abraham v. Woburn, 383 Mass. 724, 728 (1981). The Abraham court, supra at 728, quoting from Yalenezian v. Boston, supra, concluded in interpreting G. L. c. 269, § 8, that the phrase “riotously or tumultuously assembled” should be read “conjunctively to describe the offence of an unlawful assembly which has proceeded to execute an unlawful purpose in a way that has resulted in the destruction of property or [in] injury thereto, and in a manner to give firm and courageous persons in the neighborhood of such assembly reasonable grounds to apprehend a breach of peace in consequence of it.” Under the statute at issue in Abraham and Yalenezian, however, damages can only be obtained in the event the property damage is caused by the actions of “persons who are riotously or tumultuously assembled,” i.e., who are actually engaged in rioting. The holding, thus, is not necessarily determinative of the offense in question here, where property damage is not involved and the *579defendant was not charged with rioting, but rather with failing to obey an order to disperse from an “unlawful assembly.”
Second, the defendant improperly assumes that only the crime of riot is encompassed within G. L. c. 269, §§ 1, 2. Although riot is among the offenses included within G. L. c. 269, §§ 1,2, the statute refers to “rioters or persons unlawfully assembled” (emphasis supplied). G. L. c. 269, § 2. The use of the disjunctive “or” in §§ 1 and 2 supports the conclusion that unlawful assembly is a separate offense from that of rioting. See Commonwealth v. Davie, 46 Mass. App. Ct. 25, 27 (1998), quoting from Eastern Mass. St. Ry. v. Massachusetts Bay Transp. Authy., 350 Mass. 340, 343 (1966) (“The word ‘or’ is given a disjunctive meaning unless the context and the main purpose of all the words demand otherwise”). Moreover, as will be discussed more fully below, unlawful assembly historically has been an offense separate from the offense of riot.
Constitutional claim. At bottom, the defendant raises a facial challenge to the statute on grounds of overbreadth and vagueness. We will first set out relevant constitutional principles, then examine the meaning of the statutory provision in question, and, finally, consider the constitutionality of the provision in respect to this claim.
a. Relevant constitutional principles. A law is overbroad “if it uses means which sweep unnecessarily broadly and thus invades the area of protected freedoms.” Smith, Criminal Practice & Procedure § 6, at 10 (2d ed. 1983). See Broadrick v. Oklahoma, 413 U.S. 601, 613 (1973); Commonwealth v. Orlando, 371 Mass. 732, 733 (1977). We will consider the facial validity of an offense even though the defendant’s conduct4 “might be of the class properly the subject of State regulation, for ‘[i]t matters not that the [form of expression] used [by the defendant] might have been constitutionally prohibited under a narrowly and precisely drawn statute.’ ” Commonwealth v. A Juvenile, 368 Mass. 580, 584-585 (1975), quoting from Gooding v. Wilson, 405 U.S. 518, 520 (1972). “This exception [to the traditional rule of standing] is based on an overriding interest in *580preventing any ‘chill’ on the exercise of First Amendment rights.” Commonwealth v. Bohmer, 374 Mass. 368, 373 (1978).
We note, however, that because facial challenges greatly increase the number of persons who have standing to bring a claim, the United States Supreme Court has admonished that the overbreadth doctrine be employed sparingly. Broadrick v. Oklahoma, 413 U.S. at 610. See Commonwealth v. Provost, 418 Mass. 416, 422-423 (1994). If a statute’s deterrent effect on protected expression is not “both real and substantial” and if the statute is “readily subject to a narrowing construction,” the doctrine of overbreadth may not be employed. Young v. American Mini Theatres, Inc., 427 U.S. 50, 60 (1976). See Broadrick v. Oklahoma, 413 U.S. at 613.
The vagueness doctrine, in its primary sense, relates to the requirement, grounded in concepts of due process, that “[a] law is unconstitutionally vague if it is not sufficiently explicit to give clear warning as to proscribed activities.” Commonwealth v. Orlando, 371 Mass. at 734. To satisfy constitutional requirements, laws must be susceptible to ready understanding by “men of common intelligence.” Commonwealth v. Gallant, 373 Mass. 577, 580 (1977), quoting from Connally v. General Constr. Co., 269 U.S. 385, 391 (1926). In applying this standard, courts are ever mindful of “the practical difficulties in drawing criminal statutes both general enough to take into account a variety of human conduct and [yet] sufficiently specific to provide fair warning that certain kinds of conduct are prohibited.” Commonwealth v. Gallant, supra, quoting from Colten v. Kentucky, 407 U.S. 104, 110 (1972). Ultimately, however, “[n]o one may be required at peril of life, liberty or property to speculate as to the meaning of penal statutes.” Lanzetta v. New Jersey, 306 U.S. 451, 453 (1939). See Chicago v. Morales, 527 U.S. 41, 57-58 (1999) (what is illegal must be clear or due process is violated; statute must distinguish between innocent conduct and conduct threatening harm).
In addition to fixing minimum standards for providing notice of the conduct proscribed by particular laws, the vagueness doctrine also prohibits such imprecision as might give rise to arbitrary enforcement of laws. Consistent with this principle, legislatures are constitutionally required to set forth minimum *581guidelines for the enforcement of criminal statutes to avoid “a standardless sweep [that] allows policemen, prosecutors, and juries to pursue their [own] personal predilections.” Commonwealth v. Williams, 395 Mass. 302, 304 (1985), quoting from Smith v. Goguen, 415 U.S. 566, 575 (1974). See Commonwealth v. Gallant, 373 Mass. at 580 (“[a] vague law impermissibly delegates basic policy matters to policemen, judges, and juries . . .”). It is this aspect of vagueness that is implicated by the defendant’s claims here. The gravamen of the defendant’s argument with regard to vagueness is that § 2, by authorizing police to issue and enforce a dispersal order in the context of any “unlawful[], riotous[] or tumultuous[]” assembly (c. 269, § 1), confers too much discretion on police and thus potentially impinges on the rights to assemble and freedom of speech.
An additional principle to be noted is that “[wjhere a statute’s literal scope ... is capable of reaching expression sheltered by the First Amendment, the [vagueness] doctrine demands a greater degree of specificity than in other contexts.” Commonwealth v. Gallant, 373 Mass. at 580, quoting from Smith v. Goguen, 415 U.S. at 573. Cf. Commonwealth v. A Juvenile, 368 Mass. at 584, quoting from Speiser v. Randall, 357 U.S. 513, 525 (1958) (“any statute which regulates speech requires the strictest of our scrutiny because ‘the line between speech unconditionally guaranteed and speech which may legitimately be regulated, suppressed, or punished is finely drawn”).
We note also that in clarifying the meaning of a law, our discretion is limited. For example, we are required to presume that every enactment of the Legislature is intended to comply with constitutional constraints. See Commonwealth v. Welch, 444 Mass. 80, 99-100 (2005). Whenever possible, therefore, a constitutional construction must be imposed on a challenged statute. See Commonwealth v. Templeman, 376 Mass. 533, 535-536 (1978). As the Supreme Judicial Court stated in Commonwealth v. Lammi, 386 Mass. 299, 301 (1982), quoting from Commonwealth v. O’Neil, 233 Mass. 535, 540-541 (1919), we must “indulge every rational presumption in favor of [a statute’s] validity and ‘it will not be denounced as contrary to the Constitution unless its language is so clear and explicit as to render impossible any other reasonable construction.’ ”
*582Finally, in reviewing the defendant’s claim, we take note of two landmark cases involving the right of assembly under the First Amendment to the United States Constitution, Edwards v. South Carolina, 372 U.S. 229 (1963), and Cox v. Louisiana, 379 U.S. 536, 549-552 (1965). In Edwards, the United States Supreme Court, holding that the right of assembly had been infringed, overturned the convictions under a breach of the peace statute of members of a group of college students who had assembled on public property to protest discrimination against African Americans. Critical to this decision was the Court’s conclusion that there had been no obstruction of traffic and no violence or threats of violence. Id. at 231-232, 236.
In Cox v. Louisiana, 379 U.S. at 545-547, a group of protesters had peaceably assembled at a courthouse to protest segregation. Although a sidewalk across the street from the courthouse had been obstructed during part of the assembly, the Supreme Court overturned the defendants’ convictions, holding that the gathering was constitutionally protected because local officials had regularly permitted parades and similar meetings even where they had the effect of obstructing traffic, and because there was no evidence of violence or words inciting violence. Id. at 550-551, 556-557. Cf. Commonwealth v. A Juvenile, 368 Mass. at 587-599 (disorderly person offense construed to relate exclusively to activities that do not involve protected exercise of First Amendment right); Commonwealth v. Jarrett, 359 Mass. 491, 493, 496-498 (1971) (disturbing the peace offense construed to exclude protected expression).
b. Offense of refusing to disperse from unlawful assembly. With these principles in mind, we proceed to consider the defendant’s claim. Ordinarily, the starting point of our analysis must be the language of the statute. “[T]he words of a statute are the primary source from which we ascertain legislative purpose, and when the text of a statute is clear and unambiguous, we construe the language in accordance with its plain and ordinary meaning.” Commonwealth v. Hatch, 438 Mass. 618, 624 (2003). See Commonwealth v. Ray, 435 Mass. 249, 252 (2001), and cases cited; Foss v. Commonwealth, 437 Mass. 584, 586 (2002). Here, the crime in question — refusal to disperse from an unlawful assembly — involves a statute into which the *583Legislature incorporated a common-law offense — unlawful assembly — without defining that offense; we are therefore required to look to the common law for the definition of that offense. See Commonwealth v. Webster, 5 Cush. 295, 303-304 (1850); Commonwealth v. Slaney, 345 Mass. 135, 138 (1962); Commonwealth v. Jarrett, 359 Mass. at 493-495. See also Commonwealth v. Brasher, 359 Mass. 550, 553-554 (1971); Commonwealth v. A Juvenile, 368 Mass. at 585.
The pertinent Massachusetts statutes, now codified at G. L. c. 269, §§ 1 and 2, were originally enacted in 1750 and 1786, respectively. At early common law, unlawful assembly generally meant a group of persons who assembled for the purpose of rioting, but who, after meeting, did not riot or take any overt act toward carrying out their purpose. See Regina v. Puch, Holt KB 636, 87 Eng. Rep. 900 (1704); Model Penal Code § 250.1 commentary (1980); Annot., What Constitutes Offense of Unlawful Assembly, 71 A.L.R. 2d 875, § 2 n.2 (1960 & Supp. 1991). Unlawful assembly historically was one of a series of three related offenses against the public peace — unlawful assembly, rout,5 and riot. See Model Penal Code § 250.1 commentary; Commonwealth v. Gibney, 2 Allen 150, 152 (1861) (“The distinction in criminal treatises, in the definitions of riots, routs and unlawful assemblies, assumes that there must be an assembling together, and an unlawful assembly . . .”). See also Commonwealth v. Runnels, 10 Mass. 518, 519 (1813) (not necessary to use phrase “in terrorem populi” in indictment for riot, as that phrase was only necessary “in indictments for that species of riots which consist in going about armed, etc., without committing any act . . . because th[at] offense consists in terrifying the public” [referring to offense of unlawful assembly], whereas if actual riotous acts are committed those words are unnecessary).
Early Massachusetts cases referring to unlawful assembly usually involved indictments for riot and not unlawful assembly per se, but referred to unlawful assembly because it was a necessary component of the crime of riot. See Commonwealth v. Runnels, supra (indictment charging riot alleged that defendants “assembled unlawfully,” “with force and arms,” and “being so *584assembled . . . , unlawfully, riotously and routously” rushed into a public hall and disrupted voting); Commonwealth v. Gibney, supra (indictment for riot requires allegation and proof of unlawful assembly); Commonwealth v. Frishman, 235 Mass. 449 (1920). Since unlawful assembly was a necessary component or element of the crime of riot, those cases contain the definition of unlawful assembly.
For example, in Commonwealth v. Gibney, 2 Allen 150, 151 (1861), the Supreme Judicial Court observed: “The distinction in criminal treatises, in the definitions of riots, routs and unlawful assemblies, assumes that there must be an assembling together, and an unlawful assembly; although the assembly may not have been unlawful on the first coming together of the parties, but becomes so by their engaging in a common cause, to be accomplished with violence and in a tumultuous manner.” In Commonwealth v. Frishman, supra, a case involving a riot, the Supreme Judicial Court stated that the trial judge had “accurately instructed the jury upon the issues of law presented,” id. at 455, where the judge “in substance instructed the jury that riot could be described generally as a tumultuous disturbance by a sufficient number of persons . . . assembled with intent to commit or assist in the execution of an unlawful act by force and violence', that the essential element was that there must be violence and the intent to commit an act of violence; . . . that the purpose might be formed suddenly; that, although the assembly might not have been unlawful on the first coming together of the parties, it might become so by their engaging in a common cause to be accomplished through force and violence; . . . that mere presence was not enough; that there must be the intent to accomplish unlawfully the purpose by force and violence . . .” (emphases supplied). Id. at 451.
In Ela v. Smith, 5 Gray 121, 135 (1855), the Supreme Judicial Court indicated that where a gathering is poised to engage in violence, it may be denominated an unlawful assembly: “[T]he evidence tends to prove, that an unlawful assembly or mob was threatened, and that it was in view of the imminent danger to the public peace, and an anticipated violence and resistance to the laws, that the acts charged in the declaration were committed” (emphasis supplied). Thus, Massachusetts cases have *585consistently read “unlawful assembly” in the context of a gathering, the unlawful character of which derives, at least in part, from its members’ common intent to resort to force or violence, shown either by an actual resort to violence or by acts giving probable cause to believe that such violence is imminent. See Commonwealth v. Runnels, supra; Ela v. Smith, supra; Commonwealth v. Gibney, supra; Commonwealth v. Frishman, supra. See also Yalenezian v. Boston, 238 Mass. at 543 (1921) (“Persons having lawfully come together and being lawfully together, may, thereupon, become an unlawful assembly and commit a riot, although they had not that purpose when they assembled”).
Consistent with this long line of Massachusetts decisions, we conclude that the term “unlawful assembly” should be defined, for the purposes of G. L. c. 269, § 2, as any gathering otherwise meeting the requirements of that provision, the members of which have formed a common intent to “ engage [] in a common cause ... to be accomplished with violence and in a tumultuous manner” (Commonwealth v. Gibney, 2 Allen at 152) or “through force and violence” (Commonwealth v. Frishman, 235 Mass. at 352, 355),6 that is, where there is an “imminent danger . . . [of] violence.” Ela v. Smith, 5 Gray at 135.7 Thus, an “essential element” of both “riot” and “unlawful assembly” is *586“the intent to commit an act of violence.” See Commonwealth v. Frishman, 235 Mass. at 451, 455.
This construction avoids any question of constitutional infirmity; only peaceful assemblies, not violent gatherings, are protected by the First Amendment and art. 19. See Edwards v. South Carolina, 372 U.S. at 234-236; Cox v. Louisiana, 379 U.S. at 544-552.8 In this way, we satisfy our responsibility, described above, to impose a constitutional reading on § 2 if possible.
Because we conclude that the statute is constitutional, and because no transcript of the trial was submitted on appeal and we do not know on what basis the defendant was convicted, the judgment must be affirmed.
Judgment affirmed.
Although the defendant did not include a transcript of the trial proceedings on appeal, he included copies of jury instructions, which he and the Commonwealth agreed at oral argument were the instructions given at trial.
The defendant was found not guilty of two other charges, namely, disturbing the peace, G. L. c. 272, § 53, and obstructing a public passageway or sidewalk in violation of a municipal ordinance.
See now G. L. c. 269, § 1, as amended through St. 2004, c. 348, § 1.
Here, because of the defendant’s decision not to provide a transcript on appeal, we do not know exactly the conduct in which the defendant engaged.
Rout is no longer recognized as a separate crime.
There must also be the common or shared intent “mutually to assist one another against any person who should oppose them in doing so,” Commonwealth v. Gibney, supra at 152.
Other States have adopted or followed a similar approach in defining “unlawful assembly,” requiring violence or a threat of violence as part of the gravamen of the offense. See, e.g., State v. Elliston, 159 N.W. 2d 503, 505-508 (Iowa 1968) (statute defining unlawful assembly as “[w]hen three or more persons in a violent or tumultuous manner assemble together to do an unlawful act, or, when together, attempt to do an act, whether lawful or unlawful, in an unlawful, violent, or tumultuous manner, to the disturbance of others . . .”); Koss v. State, 217 Wis. 325, 326-330 (1935) (statute providing, “Any three or more persons who . . . being together, . . . make any attempt or motion towards doing a lawful or unlawful act in a violent, unlawful or tumultuous manner to the terror or disturbance of others, shall be deemed an unlawful assembly . . .”).
As Massachusetts cases cited supra, as well as Black’s Law Dictionary 124 (8th ed. 2004) point out, it is not a necessary element of “unlawful assembly” that the object sought to be accomplished be unlawful. “In order that the assembly may be ‘unlawful,’ it is not necessary that the object of the meeting should itself be illegal. The test is, not the illegality of the purpose for which *586the persons are met, but the danger to the peace which their meeting involves. The mere fact. . . that the purpose is unlawful is not enough. . . . Thus, if a number of persons meet to plan a fraud, they may be guilty of a conspiracy, but their meeting is not an unlawful assembly." Black’s Law Dictionary 124, quoting from 4 Stephen’s Commentaries on the Laws of England 135-136 (L. C. Warmington ed. 1950).
To the extent that cases concerning unlawful assembly can be read as requiring only a fear that participants will engage in riotous or violent conduct, decisions from the United States Supreme Court as well as the Supreme Judicial Court indicate that, in order to meet constitutional standards protecting the right to peaceably assemble and freedom of speech, some actually threatening conduct or an actual act or acts of violence or riotous behavior must be shown to support the dispersal order and a conviction under the statute; in other words, subjective fear on the part of witnesses or police officers is not enough, nor is loudness enough. See Edwards v. South Carolina, supra; Cox v. Louisiana, supra. Cf. Commonwealth v. A Juvenile, 368 Mass. at 596-597, 599.
Whether, in any particular case, an assembly was on the verge of rioting or violence or had formed a common intent to engage in violence at the time § 2 was invoked by police potentially may raise a difficult question of fact for the jury. While it is necessary to afford police some latitude in determining whether the requisite predicates for application of § 2 are present, police discretion cannot be construed so broadly as to undercut the right of peaceable assembly. Thus, in cases where inchoate violence forms the basis for an alleged violation of § 2, the jury charge must emphasize the necessity of a finding of a reasonable belief on the part of police based on specific conduct and actions of the assembly that a common intent to commit violence had been *587formed and that violence was imminent. Mere speculation on the part of police will not suffice to sustain a conviction. Where such factual questions are present, an instruction describing the legal standards to be applied, including reference to relevant constitutional protections, should be provided. See, e.g., Tatro v. Kervin, 41 F.3d 9, 16-17 (1st Cir. 1994).