Three police officers have been shot and wounded and more than 20 injured in rioting in Belfast, the capital of Northern Ireland.
The trouble broke out just before one of the main days of the Protestant marching season. It began at about 11.45pm (BST) on Sunday.
The BBC reports the police used water canon and fired baton rounds at the rioters.
At least one car was hijacked and set alight.
The marches commemorate Prince William of Orange's victory in the Battle of the Boyne over Catholic King James II in 1690.