Rioting broke out across Belfast after hundreds of loyalists took to the streets to protest over flags.
Three people were arrested in connection with the disorder. PSNI assistant chief constable Will Kerr said 21 police officers had been injured during violent skirmishes this week.
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 Police Service of Northern Ireland 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. Two cars were also set on fire. There were also reports of minor sporadic disorder in other parts of the city. Two PSNI water canon were deployed to the area but were not used against the rioters.
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.
On Friday 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 Thursday night police clashed with loyalists in Ian Paisley's home town of Ballymena, Co Antrim. Meanwhile, there are plans for a major protest in Belfast city centre on Saturday. Traders are worried it could affect business on what should be one of the busiest shopping days of the year.
During her visit US Secretary of State Hillary Clinton appealed for the violence to stop.
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.