Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 33, § 41

(a) The commander-in-chief may issue an order directed to the commander of any organization or unit of the armed forces of the commonwealth directing the command, or any part thereof, to appear at a time and place specified in the order to aid the civil authority in suppressing violations of law, preserving order, affording protection and supporting the laws if 1 of the following situations occurs: (i) in the case of a tumult, riot, mob or body of persons acting together by force to violate or resist the laws of the commonwealth or when such tumult, riot or mob is threatened; (ii) in the case of public catastrophe or natural disaster; (iii) if the usual police provisions are inadequate to preserve order and afford protection to persons and property; or (iv) if additional support to civilian law enforcement is necessary. The order may be issued upon the initiative of the commander-in-chief, or at the request of the sheriff of a county, the mayor or city manager of a city or the selectmen of a town.

(b) Military police forces of the national guard, both army and air, may exercise all the powers of constables, except the service of civil process, and of police officers and, when ordered to perform law enforcement duties, shall appear for duty armed and equipped: (i) at all times upon all land and buildings held for military purposes and all other military property of the commonwealth; or (ii) when on duty within the commonwealth under this chapter or Title 32 of the United States Code; provided, however, that all such military police forces shall hold the appropriate law enforcement occupational specialty as certified by the armed forces of the United States.