A group of demonstrators broke into the headquarters of Britain's governing Conservative Party in London on Wednesday, spray-painting anarchy symbols and setting off flares before being forced out of the building.
They went on to set fires outside the building.
The violence came during a largely peaceful protest by students against government plans to allow universities to increase tuition fees. The National Union of Students said 40,000 demonstrators were on the streets.
Extra police officers were sent to deal with the violence, London's Metropolitan Police said.