Police officers in Northern Ireland fired plastic bullets and used water cannon after coming under attack from rioters for a fifth consecutive night on Monday, AFP reports.
Intermittent disorder has blighted Belfast since the city council's decision on December 3 to limit the number of days it flies the British flag, or Union flag, above the City Hall.
Around 1,000 loyalist protesters held a peaceful demonstration outside the City Hall on Monday as councillors held their first meeting since last month's decision.
But trouble erupted as a group of around 250 arrived at a known boundary between loyalist and republican neighbourhoods after leaving the City Hall protest.
Police battled to keep the two groups apart, firing plastic bullets and using water cannon after coming under fire from a hail of masonry and petrol bombs on the city's Newtownards Road.
Northern Ireland's chief police officer Matt Baggott earlier accused the paramilitary Ulster Volunteer Force (UVF) of orchestrating the violence.
Loyalists believe last month's ruling to fly the flag on certain designated days was a concession too far to republicans who want Northern Ireland to be part of Ireland.
The first of these days will be on Wednesday to mark the birthday of the Duchess of Cambridge.
Elected representatives from political parties on both sides have received death threats, the latest being SDLP Assembly member Patsy McGlone.
Nationalist party Sinn Fein's Jim McVeigh said politicians "won't be intimidated by those threats".
"Their protests are pointless and they will have absolutely no impact on decisions that we take," he added.
The flag vote has raised tensions in the province, which was torn apart by three decades of sectarian violence until peace accords in 1998 led to the creation of a power-sharing government between Protestants and Catholics.
More than 60 police officers have been injured and around 100 people arrested since the row began.