The British flag was hoisted over Belfast's City Hall on Wednesday for the first time since its removal a month ago 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 that it would no longer fly the British flag all year round at the City Hall.
The emblem will now fly for a maximum of 18 days a year including British royal birthdays -- the first of which fell on Wednesday as Prince William's wife Catherine turned 31.
Its 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 1998 peace accords 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.
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 3,000 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 violence.
The 1998 Good Friday peace agreement brought an end to most of the violence, but sporadic bomb threats and murders by dissident republicans continue.