Twenty-nine Northern Ireland police officers were injured while trying to quell sectarian clashes in Belfast, as the row over limits to the flying of the British flag showed no sign of abating.
Police used water cannon and fired six plastic bullet rounds during the clashes, the latest to blight the British province after more than five weeks of violence.
A police spokesman confirmed that some of the officers required hospital treatment for injuries sustained during disorder in east Belfast.
Police Service of Northern Ireland Chief Constable Matt Baggott paid tribute to his officers.
"My colleagues brought the situation under control with exceptional courage and professionalism," he said.
"I know the vast majority of people will be grateful for their efforts," he added. "Police will continue to engage with all those committed to finding a solution to these issues."
Witnesses said rocks and fireworks were thrown as lines of police tried to keep apart loyalists - the Protestant community's working-class hardcore - and Catholic nationalists in the Short Strand area of Belfast.
Earlier, nearly 1,000 people had gathered outside Belfast City Hall to protest.
The trouble erupted around 2.30pm (local time) as the group passed the republican neighbourhood.
Northern Ireland has been swept with a wave of sometimes violent protests since December 3, when Belfast City Council voted to restrict the number of days the British flag is flown at City Hall to 18 per year.
Loyalists see the council's decision as an attack on their identity and an unacceptable concession to republicans seeking a united Ireland.
First Minister Peter Robinson and his deputy Martin McGuinness will join Northern Ireland Secretary Theresa Villiers and Ireland's Tanaiste (deputy leader) Eamonn Gilmore for talks next week on the unrest.
"This violence is being orchestrated and those behind it are known criminals, intent on creating chaos," Gilmore said.
"This has nothing to do with real issues around flags and identity in a shared society, which are the subject of intensive political discussions at present."
Nationalist SDLP MLA lawmaker Conall McDevitt said "these are depraved acts which immediately dismiss any claim on a protest being peaceful."
Alliance MLA member Judith Cochrane added: "This violence cannot continue. People want to go about their daily lives, but are really angry at the disruption they have faced in recent weeks.
"These protests and violence are doing untold damage to traders and businesses and Northern Ireland's image is being tarnished."
A 1998 peace agreement brought an end to three decades of sectarian violence between Protestants and Catholics known as the Troubles, but sporadic bomb threats and murders by dissident republicans continue.