Well, to turn this upside down, a police riot works thusly (info cited from Kristian Williams' book Our Enemies In Blue: Police and Power In America):
Williams writes, “...a breakdown in command, when paired with the widespread and excessive use of force, is perhaps the defining mark of the classic police riot.” 
II. Sociological analysis, first part
Williams writes, “In his book, Police Riots: Collective Violence and Law Enforcement, sociologist Rodney Stark offers a six-step outline as to how these riots unfold:
(1) 'Convergence'—There must be substantial numbers on both sides.
(2) 'Confrontation'—Either police actions attract hostile crowds, or police deem some gathering illegal and move in to break it up.
(3) 'Dispersal'—Police attempt to break up the crowd.
(4) 'The Utilization of Force'—Police use force against the crowd.
(5) 'The Limited Riot'—Excessive or punitive force ends once the crowd is dispersed. The limited police riot is often signified by the disintegration of police formations into small autonomous groups, charging into crowds, chasing fleeing individuals, and beating people up.”
(6) 'The Extended Police Riot'—Attacks continue even after the crowd has dispersed. Extended riots are most common in densely populated areas, like college campuses or urban ghettos. Then, police attacks often attract new crowds, thus renewing confrontations.” 
III. Sociological analysis, second part
Williams writes, “There are a number of factors that, in the right circumstances, give police actions this trajectory. Among them are specific crowd control tactics, operational deficiencies, the machismo inherent to cop culture, and a paranoid ideology that leads police to overestimate the threat crowds pose.”