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 depending on the 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.