Dozens of police were injured in Northern Ireland's capital Belfast on Friday night in clashes with pro-British Protestants protesting against a parade by Catholic republicans.
Some 56 police officers were injured, four requiring hospital treatment, when they were attacked with bricks, bottles and other missiles by crowds in the city centre, police said on Saturday.
Several cars were set on fire, while police used water cannon and fired baton rounds to try to disperse the rioters.
Seven people were arrested for offences ranging from riotous behaviour to hijacking, and Northern Ireland police chief Matt Baggott warned that many more arrests would follow.
"Those people had no intention of peaceful protest," he said.
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"They lack self respect and they lack dignity."
The city's prisons would be "bulging" once all the culprits were rounded up, he said.
Belfast is currently hosting thousands of former police and fire officers from around the globe, who are in town for the World Police And Fire Games.
Protestant loyalist protesters had tried to block part of the route of a planned republican parade marking the anniversary of the introduction by British authorities of internment without trial on August 9, 1971.
It was one of the most controversial policies of The Troubles, the three decades of civil unrest in Northern Ireland between pro-British Protestants and Catholic republicans favouring a united Ireland.
Nearly 2,000 people were held without trial under the policy, the vast majority of them republicans.
Internment lasted until 1975. It was intended to restore order in the British province, but the deeply unpopular policy ultimately boosted recruitment to the paramilitary Irish Republican Army.
There were also clashes on Thursday night at an anti-internment bonfire near Belfast city centre, when eight police officers were injured and eight people were arrested.
Last month the city was hit by several nights of rioting, mainly by loyalist groups.
More than 3,500 people died during Northern Ireland's three decades of sectarian strife.
The 1998 Good Friday agreement, which set up a power-sharing government between republicans and loyalists, largely ended the violence, although sporadic attacks and bomb threats continue.
