Dozens of police officers were injured during riots by pro-British Protestants in Belfast, in what Northern Ireland's police chief condemned on Saturday as "mindless anarchy".
A total of 56 officers were hurt, four requiring hospital treatment, when they were attacked with bricks, bottles and paving stones from the street on Friday night, police said.
Several cars were set on fire as about 1,200 people gathered in the city centre to protest against a march by republican Catholics, and police responded with water cannon and baton rounds.
At least two members of the public were also hurt, police said, and Michael Copeland, a Protestant politician, claimed he and his family were assaulted by police on the street.
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. They lack self respect and they lack dignity," he said, adding: "We saw what I can only describe as a mindless anarchy."
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 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, when Protestants were prevented from holding a parade through a republican area of north Belfast.
Tensions have risen again this weekend ahead of a march on Sunday by republicans commemorating IRA members who died during The Troubles.
Mike Nesbitt, leader of the Ulster Unionists, the second largest Protestant party in Northern Ireland, condemned Friday night's violence and said it only played into republican hands.
"Violence against the police from those belonging to the Protestant unionist loyalist community is not only wrong, it is also playing into the hands of Irish republicanism," he said in a statement.
"It removes the focus from the offensive nature of their attempt to re-write history and excuse the terrorism that blighted our country for decades, and still does."
More than 3,500 people died during three decades of sectarian strife during the 1970s, 1980s and 1990s in the British-controlled province.
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.