At least eight police officers have been injured as rioting broke out in Belfast in the latest flare of violence in Northern Ireland.
Tensions have risen in Northern Ireland since Belfast's council voted on Monday not to fly the British flag all year round, angering Protestant loyalists who believe Northern Ireland should retain strong links to Britain.
Police clashed with hundreds of loyalists close to the city centre on Friday night and five people were arrested, the Police Service of Northern Ireland (PSNI) said.
"This behaviour is unacceptable. These people are wrecking their own communities and putting lives at risk," said Assistant Chief Constable Will Kerr of the PSNI.
"This mob violence cannot continue. I am urgently appealing to politicians and those with influence to do what they can to put a stop to this."
Two cars were set alight while eyewitnesses said protesters hurled stones, bricks and bottles at the police.
Loyalists have held nightly protests in several parts of Northern Ireland since councillors ruled that the British flag can only fly above Belfast's City Hall for a maximum of 17 days a year.
Police said some 1,000 people rioted on Monday leaving 15 police officers injured, and Belfast legislator Naomi Long received a death threat on Friday for her non-sectarian Alliance party's support for the change in flag policy.
Two bombs were also found in other parts of Northern Ireland in a sign of the lingering sectarian tensions despite the peace process, which largely ended three decades of sectarian violence in the 1990s.
The violence took place just hours after US Secretary of State Hillary Clinton visited the troubled British province urging peace.
"There can be no place in the new Northern Ireland for any violence. Any remnants of the past must be quickly condemned," she told a press conference in Belfast.
With more protests expected in the city on Saturday, Clinton urged: "People have strong feelings, but you must not use violence as a means of expressing those strong feelings."
The flag, signifying Northern Ireland's links with Britain, has been flying on top of the building since 1906.
After the Irish War of Independence in 1922, the six provinces of Northern Ireland remained with Britain, while the former southern Ireland became an independent state - now the Republic of Ireland.
Monday's decision by the council prompted angry protests from Loyalists - supporters of the pro-British Protestants in the province - who stormed the building. Daily protests and clashes with Catholics followed.
They took an unpleasant turn when Protestants directly targeted the homes and offices of members of the small, cross-confessional Alliance Party, whose votes secured the passage of the flag rules initiated by Catholics on the city council.
Some 3,600 people died in the three decades of violence between Northern Irish Protestants favouring continued union with Britain, and Catholics seeking a unified Ireland.
A 1998 peace agreement largely ended the conflict, but sporadic unrest and bomb threats continue as dissident offshoots remain violently opposed to the power-sharing government in Belfast, formed of Catholic and Protestant parties.