Needs five elements: (a) it must involve at least 12 persons (three at common law, 12 by the Public Order Act 1986 making it a criminal act); (b) all must have a common purpose; (c) there must have been an inception or execution of that common purpose; (d) they must intend to help one another by force if necessary against any person who may oppose them in the execution of their common purpose; (e) force or violence must be used in such a manner as to alarm at least one person of reasonable firmness and courage. This definition originated in Field v. Receiver of the Metropolitan Police (1907). Riot is often insured as an additional peril (linked with malicious persons) under property insurances.
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One of the extended coverage perils, related to, but broader than, civil commotion.