The British flag was hoisted over Belfast's City Hall on Wednesday for the first time since the decision not to fly it permanently sparked riots in Northern Ireland.
On a sixth consecutive night of violence in the British province, protesters pelted police in the capital Belfast with petrol bombs, fireworks, bottles and stones.
Pro-British protesters have taken to the streets almost every night since December 3, when the city council announced it would no longer fly the Union Jack all year round at the City Hall.
It will now only be hoisted for a maximum of 18 days a year, including on the birthdays of British royals - the first of which fell on Wednesday as Prince William's wife Catherine turned 31.
The flag's reappearance above the elegant central Belfast building raised fears of more violence as protesters vowed to continue their campaign until it is replaced permanently.
The flag ruling sparked riots and arson attacks at the start of December which gave way to largely peaceful protests, but the violence has flared again since the start of the new year.
Tensions are running high in the province, which endured three decades of sectarian violence until peace accords in 1998 led to a power-sharing government between Protestants and Catholics.
The protesters, who are mainly Protestant, see the flag's removal as an attack on their British identity and a compromise too far with republicans, who are mostly Catholic and favour a united Ireland.
British Prime Minister David Cameron said on Wednesday that Northern Ireland needed to break down "barriers of segregation that have been in place for many, many years".
"We need to build a shared future in Northern Ireland," he said as he faced his weekly session of questions in parliament.
"I think that is part of the challenge to take away some of the tensions that we've seen in recent days."
John Kyle, a member of the pro-British Progressive Unionist Party on the city council, said the protests expressed the wider anger of Protestants who feel they have lost out in the peace process.
"There's a feeling of alienation - they feel disconnected from the political system," he told BBC radio.
"It has erupted in this anger and regrettably the anger has led to violence."
Some 3000 people were killed in the three decades of sectarian bombings and shootings from the late 1960s known as "The Troubles".
Northern Ireland's top policeman Matt Baggott has accused the paramilitary Ulster Volunteer Force, which murdered more than 500 people during the conflict, of orchestrating some of the recent violence.
The 1998 Good Friday peace agreement brought an end to most of the unrest in the province, but sporadic bomb threats and murders carried out by dissident republicans continue.