Fifty-six police officers and two civilians were injured in violent riots between loyalists and nationalists in central Belfast on Friday night.
The disorder broke out as Northern Ireland's Protestant community held counter-protests against a parade by Catholic nationalists, according to CNN.
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Police fired plastic bullets and a water cannon after being attacked with everything from scaffolding poles to paving stones ripped up from the city's shopping district.
Five of those injured needed hospitalization, and at least seven arrests have been made so far.
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.
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Northern Ireland's top police officer, Chief Constable Matt Baggott, called the violence "mindless anarchy."
"Last night's violence and attacks on police officers were shameful," Britain's minister for Northern Ireland, Theresa Villiers, said in a statement. "Disorder on the streets is a hugely regrettable step backwards."
Police were braced for further trouble Saturday.
More parades organized by the Apprentice Boys, a Protestant group, were scheduled to take place across the country, with the main demonstration slated for Londonderry.