The Riot Act was a method of dealing with riots used by the UK armed forces until the late 1960's.
Basically a rioting crowd, after CS
gas had usually been employed without any real success, would be warned to disperse, either by megaphone or a large banner (as this always happened abroad it could be in the local language).
The warning would be to disperse of take the consequences, it would involve reading out or signifying a piece of military law called 'The Riot Act'.
The consequences were usually that a ringleader of the riot would be indentified, then shot by a marksman.
By the time of Northern Ireland from 1969, this law no longer applied, the military were under full civilian control, and developments like rubber bullets and water cannon made the act superfluous
as well as politically unacceptable.