Rioting broke out in Belfast late Friday in the latest flare of violence in Northern Ireland, just hours after US Secretary of State Hillary Clinton visited the troubled British province urging peace, AFP reports.
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.
Eight police officers were injured as they 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.
Two cars were set alight while eyewitnesses said protesters hurled stones, bricks and bottles at the police.
"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."
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.
There are plans for a major demonstration against the flag ruling in central Belfast on Saturday.
Police said some 1,000 people rioted on Monday leaving 15 police officers injured, and Belfast lawmaker 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 fresh unrest came after Clinton visited Belfast, winning praise from Northern Irish First Minister Peter Robinson and his deputy Martin McGuinness for her role in the peace process, and condemning the renewed spate of violence.
"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.
Robinson and McGuinness praised America's top diplomat and her husband, the former US president Bill Clinton, for their role in ending the bloodshed between loyalists and Catholic republicans.
Bill Clinton was a key player in the peace process during the 1990s, and the support the Clintons had given Northern Ireland in helping to bring jobs to the province would "never be forgotten", McGuinness said.
"Both Hillary and Bill Clinton have been absolutely vital voices for us in our process and that is something that has to be recognised, over many, many years."
Some 3,500 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.