Five PSNI officers were hurt after hundreds of protesters tried to storm Belfast City Hall following a vote to restrict the flying of the union flag to 15 days a year.
Police used batons and brought in dogs to push the protesters, some of whom had scarves covering their faces, out of the courtyard at the back of City Hall.
Cars belonging to council members and staff were damaged during the disorder.
A council security worker was also reported to have been hurt.
Earlier, councillors agreed by 29 votes to 21 to bring City Hall into line with Stormont and other government buildings in Northern Ireland to fly the union flag on designated days.
Just over a week ago, the council's strategic policy and resources committee voted 11-9 in favour of removing the flag completely.
The decision had to be ratified at a full meeting.
Belfast City Hall opened in 1906 and the union flag has flown from a flagpole in the centre of the building every day since.
The PSNI said two female police officers were taken to hospital. Their injuries are not believed to be life-threatening.
A spokeswoman said violence had spread to other parts of Belfast.
She said reinforcements had to be called in to deal with the hundreds of loyalists throwing missiles, including fireworks, bricks and bottles.
It is understood a press photographer was also taken to hospital for treatment to hand and facial injuries.
The situation was said to have calmed by 11pm. The PSNI deployed its helicopter and riot squad officers monitored a large crowd which had gathered at Albertbridge Road and Newtownards Road.
Hundreds of loyalists took to the streets and burnt Irish tricolours to show their opposition to the motion to have the flag removed for the first time in over a century.
Sinn Féin MLA Gerry Kelly, who sits on the Northern Ireland Policing Board, said the PSNI had serious questions to answer over the handling of the situation.
Speaking from City Hall, he said: "I have to say, and I don't use these words unless I really mean them, it was a disgraceful police operation - or lack of a police operation.
"If that had been 1,000 or more republicans, it would have been very different. There they would not have left it that they were able to come into the back of City Hall.
"They indiscriminately attacked cars. We are very, very lucky that they didn't get into the building or we could have been dealing with a lot more injuries."
Ulster Unionist Jim Rodgers said he was not surprised violence had flared.
"There is a real anger," he said. "I cannot condone violence but people do not realise just how strongly the people in Northern Ireland think about flying the flag over City Hall."
Maire Hendron from the Alliance Party said the violence was orchestrated through social media sites.
She said: "Tonight was a disaster for this city."
DUP councillor Christopher Stalford condemned those responsible for violence.
He said: "There can be absolutely no excuse for the violence which ensued after the result of the vote on the flying of the Union flag.
"The violence has distracted from the issue we should be focusing on - the parties in Belfast City Council who have voted as a block to strip the City Hall of its flag."
Northern Ireland Justice Minister David Ford condemned the violence.
He said: "The violence which took place at the City Hall and round St Matthew's Church was the responsibility of two groups of people. The first is those who went to the City Hall spoiling for a fight, who attacked police officers and council staff.
"But there is a second group which bears responsibility. DUP and UUP politicians fomented this protest, with both leaflets and the use of social media.
"They called people on to the streets. They must have known, from experience as recent as this summer, that violence was almost inevitable. They cannot avoid their responsibility.
"Such violence should be condemned by all civic and community leaders."