Around 2000 pro-British loyalists rallied in central Belfast for the return of the British flag to the roof of city hall after a vote by Irish nationalist councillors to remove it sparked a week of rioting.
Twenty-eight police officers have been injured in the most widespread pro-British street violence for years in the province as the flag became a rallying point for people who feel there have been too many concessions to Irish nationalists.
Rioters fired bricks and petrol bombs at police and burned out cars overnight, hours after U.S. Secretary of State Hillary Clinton called for calm during a visit to the city and warned the peace process was not yet complete.
Loyalist political parties, who share the protesters' desire to remain part of the United Kingdom, condemned the rioting as did the Irish nationalist parties with whom they share power.
Around 2000 people gathered outside the imposing 19th century Baroque city hall, most waving British flags and many hiding their faces with balaclavas or scarves, prompting some local businesses in the area to close.
The crowd cheered when one protester burned an Irish tricolour flag and sang the British national anthem before dispersing. Banners declared "Proud to be British" and "No Surrender."
"This goes on until the flag is back above city hall," said protester William Arthur. "Ulster is British and we will not stand for this".
Hundreds of riot police stood by, but did not intervene.
One police officer was injured during trouble in East Belfast as some of the crowd returned home, police said.
Assistant Chief Constable Will Kerr of the Senior Police Service of Northern Ireland said the disorder orchestrated by loyalist paramilitary groups was putting lives at risk.
"I am urging everyone to be calm, take a step back," he said.
At least 3600 people were killed over three decades as Catholic nationalists seeking union with Ireland fought British security forces and mainly Protestant loyalists determined to remain part of the United Kingdom.
A 1998 peace accord has mostly held, although militant nationalists have stepped up attacks in recent years and community relations remain fragile, with riots erupting every few months.
Monday's council decision means the British flag will be flown over city hall on 17 designated days including public holidays each year, as is the case at the provincial assembly at Stormont in the British-controlled province.
Until then, it had flown above the provincial capital's city hall every day since it opened a century ago, a symbol for many Catholic nationalists of Protestant domination.
Its removal has turned the tables, sparking fears of growing nationalist power.
"It's not just that the flag has come down, loyalists really sense that everything is about concessions," said Peter Shirlow, professor of conflict transformation at Queen's University. "Rightly or wrongly they sense that this is a one way process."
The violence, he said, was a sign that while loyalist paramilitaries have not in the past reacted violently to killings by dissident Irish nationalists, they may in future.
"It's a sign that it's getting harder to maintain the peace process within loyalism," he said. "Whether that breaks down is a different matter, but I think it's harder to hold the line."