At least 20 police officers have been injured during serious rioting during a Protestant parade through a nationalist area of north Belfast.
Petrol bombs and bricks were thrown at police by nationalists and loyalists in Ardoyne, although their injuries are not believed to be serious.
Police responded by using water cannons to contain the crowds, whilst six non-lethal baton rounds were fired by the security forces. There have been two arrests, although police expect more to follow.
The senior police officer in charge of the security operation in north Belfast, Assistant Chief Constable Will Kerr, urged individuals and communities affected to respond in a calm and responsible manner.
"Violence has serious and unwanted consequences for us all and we will robustly investigate all incidents of disorder," he warned.
It followed a day of largely peaceful parades by the Orange Order across Northern Ireland. The Twelfth of July represents the culmination of the Protestant Orange Order's marching season.