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 last 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.
Police said an attempt to hijack the van had been made.
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."
Protesters battle police in Belfast's Shaftsbury Square
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 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.
There are plans for a major protest in Belfast city centre today. Traders are worried it could affect business on what should be one of the busiest shopping days of the year.
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 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.