Violence which erupted in Belfast after a controversial vote on the Union flag was unacceptable, Northern Ireland's First Minister warned today.
Fifteen police officers, two security workers and a press photographer were injured when a mob of up to 1,000 loyalists rioted in the city centre and parts of east Belfast.
At the height of the trouble, police officers were pelted with bricks, bottles and fireworks and a large crowd tried to smash their way into City Hall.
They broke through barriers and forced their way through iron gates at the rear of the building before assaulting council security staff and attacking cars.
Peter Robinson said: "There is no excuse or justification for attacks on police officers, council staff, and property.
"Such behaviour is not representative of those who campaigned to maintain the Union flag flying over Belfast City Hall."
At one point, loyalists, who used scarves to hide their identity, also tried to kick down the back door to gain entry to City Hall.
The DUP leader said the proposal to change the council's flag-flying policy was provocative.
He added: "The decision to pursue the removal of the flag from City Hall and other council buildings, despite warnings of the likely consequential impact on community relations, was foolish and provocative.
"Those who talk most about building community relations have by their actions in the council substantially damaged relations across the city."
Later, the mob attacked St Matthew's Catholic Church in the nationalist Short Strand area of east Belfast and clashed with police at Albertbridge Road and Templemore Avenue. It is understood a bus was also hijacked.
Trouble broke out minutes after Belfast city councillors voted to remove the flag from City Hall. It is due to be taken down today for the first time since the Edwardian building opened in 1906.
Councillors agreed by 29 votes to 21 with a motion which would bring City Hall into line with Stormont and other Government buildings for the flag to be flown on 17 designated days.
Sinn Fein Policing Board member Gerry Kelly said the Police Service of Northern Ireland had serious questions to answer about their operation.
"It was a disgraceful police operation - or lack of a police operation," he said. "What we need now are arrests."
Mr Kelly said he would be raising the issue with the Chief Constable during a Policing Board meeting on Thursday.
Meanwhile, Northern Ireland Justice Minister David Ford added his condemnation of the violence but accused unionists of trying to whip up tensions.
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."
In a statement, the PSNI said three men had been arrested in connection with 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.
A spokeswoman said: "At approximately 10pm, police received a report of damage caused to a bus in the Albertbridge Road area. It is believed an attempt may have been made to hijack the vehicle.
"Bottles and other missiles were also thrown and (there were) reports of damage to homes in the Strand Walk area."