Massachusetts General Laws Part IV. Crimes, Punishments and Proceedings in Criminal Cases (Ch. 263-280) Ch. 269, § 2

Whoever, being present and being so commanded to assist in arresting such rioters or persons so unlawfully assembled, or in suppressing such riot or unlawful assembly, refuses or neglects to obey such command, or, if required by such magistrate or 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 punished by imprisonment for not more than one year or by a fine of not less than one hundred dollars or more than five hundred dollars, or both.