Unlawfully assembly is a specific legal statute in some states, including Massachusetts. It is similar to Some state laws call it refusal to disperse.
Unlawful Assembly Penalties
Unlawful assembly is a misdemeanor criminal charge, with a maximum penalty of 1 year in jail in most states and jurisdictions.
What makes an assembly lawful versus unlawful?
The law in Massachusetts says that if 5 or more people are assembled “riotously or tumultuously”, or are carrying weapons, then it may be considered an unlawful assembly.
Of course, riotous or tumultuous behavior is extremely subjective.
The right for citizens to peaceably assemble is enshrined in the first amendment of the United States Constitution, so any infringement of this right, such as arresting of protesters, should be immediately challenged in court.