Clashes in central Belfast left 56 police officers and two civilians injured in the latest flare-up in tensions between Northern Ireland's Protestant and Catholic communities.
Police fired plastic bullets and water cannon on late on Friday after being pelted with missiles for a second successive night. Many of the injuries were minor, but four officers were taken to hospital.
Trouble flared when protestants tried to block a march along the city's main thoroughfare, Royal Avenue, by the nationalist side of the community.
When police moved in to clear them, they threw bricks, bottles and fireworks. Burnt-out cars and rubble littered the city centre and shop fronts were damaged.
Police said seven people were arrested.
Assistant Chief Constable George Hamilton, who commanded Friday night's police operation, said the violence was "sheer thuggery".
"There were all sorts of weapons and equipment being used against the police including scaffolding and masonry. People were pulling up the paving stones from the busiest shopping precinct in Belfast."
The Catholic parade, marking the anniversary of the 1971 introduction of internment without trial by British authorities, eventually had to pass along a different route.
Eight were hurt on Thursday night when a crowd at a bonfire to mark the anniversary in a Catholic-dominated part of Belfast threw paint bombs, bottles and masonry at police.
Belfast remains divided between pro-British Protestants and Catholics who generally favour unification with Ireland, despite a 1998 peace and power-sharing deal that put an end to the worst of the "troubles" in the British province.