Two police officers have been injured after hundreds of loyalists tried to storm Belfast City Hall in response to councillors voting to remove the Union flag.
A mob forced its way through the back gates and tried to kick the door down minutes after elected members agreed to fly the flag only on designated days.
Police used batons and brought in dogs to push the protesters, who had scarves covering their faces, out of the courtyard at the back of City Hall on to Donegall Square South.
The female officers were injured after at least one firework, bricks, bottles and other missiles were thrown. It is also understood a council security guard was hurt. Their injuries are not believed to be life-threatening.
Cars belonging to elected members and staff were damaged during the disorder.
Sinn Fein's Jim McVeigh said: "The people that broke through the gates are a bunch of thugs. They physically assaulted the staff, tried to attack members of the police and they attacked property.
"We are not responsible for the thugs. We won't be intimidated by those people."
Councillors agreed by 29 votes to 21 to bring City Hall into line with Stormont and other government buildings and fly the flag on 17 designated days.
It is the first time the Union flag has been taken down from the Edwardian building in more than a century.
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."
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.
During an emotive debate on the issue, PUP councillor John Kyle said there was a real anger among working-class Protestants.
He claimed that recent decisions by the Parades Commission, investigations by the PSNI Historical Enquiries Team and the demonisation of flute bands had created tension.
He said: "Loyalists see their identity being steadily eroded. They feel like their identity is being taken away. It is a major issue.
"Unionists are not against change but this is different. This is too much too soon."
Belfast City Hall opened in 1906 and the Union flag has flown from a flagpole in the centre of the building every day since.
Unionists have opposed any changes and accused nationalists of trying to raise tensions.
Earlier Sam McCrory, 24, from Belfast, said he was at City Hall for a peaceful protest.
He said: "For me, flying the Union flag is not something that is up for negotiation.
"We should be able to come into our city hall to see the flag flying. I see the removal of the flag as a first step towards the breakdown of the union."
Sam Jackson, 69, a retired builder, said: "While Belfast is still part of the UK, the flag of the country should be flown. A compromise is not acceptable."
An amendment to fly the Union flag 365 days a year in the Garden of Remembrance at the west side of City Hall is to be considered next month.