Belfast has been hit by another night of rioting as hundreds of loyalists took to the streets to protest over flags.
Eight police officers were injured, with two needing hospital treatment, and five people were arrested following clashes across the city.
Police are carrying out a major security operation to prevent further violence later today.
With a major protest planned in the city's centre, traders are worried business could be seriously affected on what should be one of the busiest shopping days of the year.
The Christmas market in Belfast city has been temporarily closed.
Police vans cut off part of Belfast's Sandy Row areaTrouble 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. Two cars were also set on fire.
Two PSNI water cannons were deployed to the area but were not used against the rioters.
There were also reports of minor sporadic disorder in other parts of the city.
PSNI Assistant Chief Constable Will Kerr said: "This behaviour is simply not acceptable. These people are only wrecking their own communities and putting innocent people's lives at risk.
"Twenty-one officers have been injured so far this week dealing with this disorder. That's 21 officers who have been pulled away from their roles of protecting the community."
A burned out car in Belfast city centre after rioters dispersed"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 that the PSNI would provide a "very strong presence" over the weekend "to assist in creating a safe environment for everyone".
Loyalists opposed to a restriction on the number of days the Union flag can be flown over Belfast City Hall have held protests across Northern Ireland every night since the decision was taken, 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 and Dungannon, Co Tyrone.
Police said there were no reports of violence outside Belfast. On Friday night, police clashed with loyalists in Ian Paisley's home town of Ballymena, Co Antrim.
During her visit to Northern Ireland, US Secretary of State Hillary Clinton appealed for the violence to stop.
Hillary Clinton has condemned the recent violenceMuch 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 sectarian 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.