Longman Dictionary of Contemporary English
n. A unit of the police specifically trained to deal with riot#Verbing crowds. They use special equipment for personal defense as well as for crowd control and dispersal.
Riot police may be regular police who act in the role of riot police in particular situations or they may be separate units organized within or in parallel to regular police forces. Riot police are used in a variety of different situations and for a variety of different purposes. They may be employed to control riots as their name suggests, to disperse or control crowds, to maintain public order or discourage criminality, or to protect people or property. In some cases, riot police may function as a tool of political repression by violently breaking up protests and suppressing dissent or civil disobedience. In recent decades, observers have noted the increasing militarization of riot and protest policing, with police deploying snipers, flashbang grenades and guns with plastic bullets.