A demonstration or street protest is action by a mass group or collection of groups of people in favor of a political or other cause; it normally consists of walking in a mass march formation and either beginning with or meeting at a designated endpoint, or rally, to hear speakers. Actions such as blockades and sit-ins may also be referred to as demonstrations. Demonstrations can be nonviolent or violent (usually referred to by participants as "militant"), or can begin as nonviolent and turn violent dependent on circumstances. Sometimes riot police or other forms of law enforcement become involved. In some cases this may be in order to try to prevent the protest from taking place at all. In other cases it may be to prevent clashes between rival groups, or to prevent a demonstration from spreading and turning into a riot.