Rioting broke out across Belfast after hundreds of loyalists took to the streets to protest over flags.
Eight police officers were injured after mobs of youths draped in Union flags clashed with the Police Service of Northern Ireland riot squad close to the city centre on Friday night. A further six officers were injured as they dealt with disorder near Crumlin Road and Ligoneill Road.
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. A spokeswoman for the PSNI said: "Police can confirm that a vehicle was driven erratically in the Shaftesbury Square area during minor disturbances."
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.
Two PSNI water cannon were deployed to south Belfast. One was also used 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.
Seven men were arrested following the disorder in the O'Neill Road area.
There were also reports of sporadic disorder in other parts of the city.
PSNI assistant chief constable Will Kerr said a total of 27 police officers had been injured during the unrest this week. He said: "This behaviour is 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."
On Friday 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.