Now considerably detailed through criminal law definitions in most jurisdictions.
For example, Canada's Criminal Code, at §63, defines an unlawful assembly as:
"An unlawful assembly is an assembly of three or more persons who, with intent to carry out any common purpose, assemble in such a manner or so conduct themselves when they are assembled as to cause persons in the neighbourhood of the assembly to fear, on reasonable grounds, that they will disturb the peace tumultuously; or will by that assembly needlessly and without reasonable cause provoke other persons to disturb the peace tumultuously.
"Persons who are lawfully assembled may become an unlawful assembly if they conduct themselves with a common purpose in a manner that would have made the assembly unlawful if they had assembled in that manner for that purpose."
In R v Patterson, a 1931 case of the Ontario Court of Appeal, published at 3 DLR 267, the Court spoke as follows on the issue:
"The statute was passed to secure orderly and peaceable conduct upon the streets, and to avoid tumultuous conduct of assembled crowds which might cause actual rioting, or which, in the opinion of persons of reasonable firmness and courage, might result in public disturbance.
"The object of those who assemble may be perfectly innocent, even highly commendable, yet, if the circumstances, in the mind of the ideal, calm, courageous, and reasonable man, are such as to lead him to fear that the public peace is in danger, it is the duty of those assembled to disperse...
"No matter how worthy the cause, or how clear the right to be asserted may be, our law requires the worthy cause to be advocated and the right to be asserted in a peaceable way, and not by riot and tumult. The provision of the Code prohibiting unlawful assemblies is for the purpose of drawing the line between a lawful meeting and an assembly, either unlawful in its inception, or which is deemed to have become unlawful either by reason of the action of those assembled, or by reason of the improper action of others having no sympathy with the objects of the meeting."
In the common law, there were three varieties of offences, all on an escalating scale and which started with the unlawful assembly, then to the rout and finally, a riot.
Canada’s Criminal Code includes a little-known and very quaint section of law (§67), a literal “reading of the riot act” and a clear relic of years gone past, where in the event of an unlawful assembly of 12 or more persons, a judge, mayor, warden, sheriff or police officer:
“...shall go to that place and, after approaching as near as is safe, if the person is satisfied that a riot is in progress, shall command silence and thereupon make or cause to be made in a loud voice a proclamation in the following words or to the like effect: 'Her Majesty the Queen charges and commands all persons being assembled immediately to disperse and peaceably to depart to their habitations or to their lawful business on the pain of being guilty of an offence for which, on conviction, they may be sentenced to imprisonment for life. GOD SAVE THE QUEEN.'"