For Reference - "Police Riot":
A police riot is a riot carried out by the police; a riot that the police are responsible for instigating, escalating or sustaining as a violent confrontation; an event characterised by widespread police brutality; a mass police action that is violently undertaken against members of the public for the purpose of political repression.
The term "police riot" was popularized after its use in the Walker Report, which investigated the events surrounding the 1968 Democratic National Convention in Chicago to describe the "unrestrained and indiscriminate" violence that the police "inflicted upon persons who had broken no law, disobeyed no order, made no threat."[1] In this sense a police riot refers to rioting carried out by the police (or those acting in a police capacity) rather than a riot carried out by people who may be motivated to a greater or lesser degree by grievances with the police (see the 1981 Toxteth riots or the 1992 Los Angeles Riots for examples of riots over policing rather than police riots).
http://en.wikipedia.org/wiki/Police_riot