Twelve police officers have been hurt during one of the worst nights of violence in northern Ireland for years.
Rioting broke out as unionists were due to take part in an Orange Order parade through the Ardoyne area of north Belfast on Monday night.
Police used water cannons to try to calm the situation as a bullet was fired at them and bricks were thrown.
Nationalist political party Sinn Fein blamed the trouble on some republicans who want to disrupt the peace process.
There were also disturbances in several other towns, including Londonderry and Armagh.
Police have promised a full investigation to find out exactly who was involved.
The Twelfth of July Orange Order parades mark the victory of the Protestant Prince William of Orange over the Catholic King James II at the Battle of the Boyne in 1690.