Other forms: rioters
A rioter is someone who takes part in a brawl or a violent disturbance. If you want a peaceful protest rally, don't invite rioters.
Sometimes protesters are accused of being rioters: the difference between the two is that protesters are generally peaceful, and that even if they're stirred to violent action, protesters have a social or political goal — or an event or policy to protest. Rioters only have the goal of causing a disruption. Sports fans sometimes turn into rioters after a big loss (or win). Rioter comes from the Old French, in which it means "chatter, dispute, or quarrel."