At common law, an unlawful assembly is a gathering of at least three persons whose conduct causes observers to reasonably fear that a breach of the peace will result. Although freedom of assembly is guaranteed by the First Amendment to the Constitution, law enforcement has the right to require disbursement of such an assembly as part of the "police powers" of the state. Determination of the potential dangers of riot or breach of peace are subjective and decided on the spot by police officers or other public officials.
Claims of "unlawful assembly" were often used to break up labor union picket lines until the late 1930s, against peaceful civil rights marches in the 1950s and 1960s, and by the police against anti-Vietnam War demonstrators in the late 1960s.
The following is an example of a local unlawful assembly statute:
"Wherever three or more persons assemble with intent or with means and preparations to do an unlawful act which would be riot if actually committed, but do not act toward the commission thereof, or whenever such persons assemble without authority of law, and in such a manner as is adapted to disturb the public peace, or excite public alarm, such assembly is an unlawful assembly."