Eighteen people, including 15 police officers, have been injured during a riot at Belfast City Hall after the passing of a vote to stop flying the Union flag all year round.
The violence spread to parts of east Belfast on Monday night where loyalists who had earlier tried to smash their way into the grounds of City Hall attacked a Catholic church.
A police spokeswoman said reinforcements had to be called in to deal with hundreds of loyalists throwing missiles, including fireworks, bricks and bottles.
The Police Service of Northern Ireland (PSNI) said three men had been arrested over the disorder.
An 18-year-old was arrested in Clandeboye Street on suspicion of riotous behaviour and possession of a petrol bomb.
A 22-year-old was arrested in the Donegall Square area, and a 17-year-old was detained on suspicion of riotous assembly in east Belfast.
Trouble broke out minutes after Belfast city councillors voted to remove the flag from City Hall.
It is the first time the Union flag has been taken down from the Edwardian building since it opened in 1906.
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.
Sinn Fein Policing Board member Gerry Kelly said the PSNI had serious questions to answer about their operation.
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 out 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."
Cars belonging to elected members and council staff were damaged during the disorder.
DUP councillor Christopher Stalford condemned those responsible for violence.
He said: "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 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."