A major security operation is underway to prevent more trouble breaking out in Belfast later today.
Eight police officers were injured and 12 people arrested during overnight clashes between loyalists and riot police.
Loyalist protesters opposed to new restrictions on flying the Union flag are expected to stage another demonstration in the city centre from about 1pm.
PSNI Assistant Chief Constable Will Kerr urged those who have a legitimate right to protest to take a step back and stay out of the city centre.
“The risk to the public safety of all of those involved is simply too great,” he said.
Traders are worried it could affect business on what should be one of the busiest shopping days of the year.
A children’s walk, Christmas party and Santa planned by a charity for youngsters with heart disease have already been cancelled.
Violence erupted in towns across the North this week after loyalists took to the streets to vent their anger.
Last night a number of officers were injured as they dealt with disorder near Crumlin Road and Ligoneill Road in Belfast.
A total of 12 people have been arrested, including a 13-year-old boy.
Trouble flared at Shaftesbury Square – a popular party spot near Queen’s University – after a man tried to drive a black van through a loyalist road block of about 200 people.
Eye witnesses said police officers were pelted with stones, bricks, bottles and other missiles. A number of vehicles were also hijacked and set on fire.
PSNI water cannons were deployed, including one in O’Neill Road in Newtownabbey on the outskirts of north Belfast as a crowd of 70 people blocked the road and attacked police with bricks and masonry, PSNI said.
Cars there were also burnt outside the council offices, and hundreds of people attending a Christmas function had to be evacuated.
There were also reports of sporadic disorder in other parts of the city.
Mr Kerr said a total of 27 police officers had been injured during the unrest this week. He said the behaviour was “simply not acceptable”.
“This mob violence and intimidation cannot be allowed to continue and I am urgently appealing to politicians and those with community influence in these areas to do what they can to put a stop to this behaviour now before someone is seriously injured or killed,” he added.
“My colleagues are providing a very strong presence and robust approach to this violent disorder and I can assure members of the public that we will not tolerate any form of criminal or violent activity. Those responsible will be brought to justice.”
Loyalists opposed to a restriction on the number of days the Union flag can be flown over Belfast City Hall have held protests across the North every night this week despite appeals from the First Minister for the pickets to be suspended.
Last night demonstrations were also held in the greater Belfast area, Bangor, Co Down, Enniskillen, Co Fermanagh, Dungannon, Co Tyrone and Ballymoney, Co Antrim. Police said there were no reports of violence outside Belfast.
On Thursday police clashed with loyalists in Ian Paisley’s home town of Ballymena, Co Antrim.
The violence has overshadowed a visit to Northern Ireland by US secretary of state Hillary Clinton.
Mrs Clinton, who was a key figure in the peace process during the 1990s, appealed for calm.
Much of the anger has been directed towards the non-sectarian Alliance Party, whose councillors in Belfast voted to limit the number of days the Union flag could be flown over the city hall from 365 to 17.
A death threat against the party’s only MP, Naomi Long, marked a significant escalation in the seriousness of the tension.
On Wednesday a constituency office in Carrickfergus, Co Antrim was destroyed in an arson attack. An attempt to burn down Bangor premises occupied by the Alliance Party’s Employment Minister Stephen Farry was thwarted.
Belfast city councillor Laura McNamee was also advised to leave her home while two others living in Bangor had their house vandalised.