In the film The Hill (which I highly recommend) the sergeant-major threatens the near-rioting prisoners with “I’ll read you the riot act” to get them to calm down and in the context of the film he means the phrase literally. The threat had a significant effect on the prisoners. So what was the “riot act”?
The reading of the riot act allowed the British army to be called in to quell disturbances. To invoke the Riot Act of 1715, the following had to be read out loud:
The key to the whole notion is that once the riot act has been read, if the crowd fails to dispurse, soldiers were allowed to open fire and shoot to kill.
here’s another modern-day example (from the state of Arizona, USA)…
** "13-2902. Unlawful assembly; classification**
A. A person commits unlawful assembly by:
Assembling with two or more other persons with the intent to engage in conduct constituting a riot as defined in section 13-2903; or
Being present at an assembly of two or more other persons who are engaged in or who have the readily apparent intent to engage in conduct constituting a riot as defined in section 13-2903 and knowingly remaining there and refusing to obey an official order to disperse.
B. Unlawful assembly is a class 1 misdemeanor.
** 13-2903. Riot; classification**
A. A person commits riot if, with two or more other persons acting together, such person recklessly uses force or violence or threatens to use force or violence, if such threat is accompanied by immediate power of execution, which disturbs the public peace.
In Arizona, unlawful assembly is not, in itself, justification for deadly force, although there’s as provision for use of deadly force in reference to an armed rioter (cite: http://www.azleg.state.az.us/ars/13/00410.htm).