A gunshot has been fired at police in north Belfast as clashes broke out following Orange Order parades in the province.
A number of officers and rioters have been injured.
Irish nationalists in Ardoyne, a Catholic enclave of north Belfast, attempted to block a march by Orangemen, seen as provocative by the Catholic community.
Abuse, petrol bombs, bricks and golf balls were hurled at police and marchers.
The day typically brings tensions to the the boil as British Protestants commemorate centuries-old victories over Irish catholics.
However, hundreds of Orange marches across Northern Ireland passed off without incident.
It is the worst violence in the city since 2005, which has been relatively calm following the signing of the 1998 peace accord.