A unionist mob stormed into the grounds of Belfast City Hall and clashed with police on Monday night after the city council voted to remove the Union Flag from the building for most of the year.
The Police Service of Northern Ireland said that 15 of their officers and two security guards had been injured during an hour-long melee outside city hall.
And a press photographer was taken to hospital with a head wound and broken finger after being reportedly clubbed by a police officer.
The violence spilled into east Belfast, where crowds threw bricks and bottles at a Catholic church and hijacked a bus.
More than 1,000 unionists had demonstrated as council members voted 29-21 to remove the Union Flag from the building for all but 17 days each year.
The crowd smashed through the locked rear gates, vandalised staff cars and fought police.
Some tossed fireworks and smoke bombs at police lines and a few climbed onto police armoured cars in the street and planted Union Flags on them.
Belfast council once had a unionist majority, but a growing republican community means it now has 24 republicans and just 21 unionists.
The balance of power is wielded by six councillors from Northern Ireland's cross-community Alliance party.
Tension had been growing since republican council members proposed a motion last month to remove the flag completely from the dome of the building.
But the Alliance party won republican support for a compromise that the flag should still fly on official holidays and other specified days.
That measure has been adopted on other government buildings, notably the Stormont castle headquarters of Northern Ireland's regional government.
But there was no compromise from the unionists.
"This is a very hot issue.
"Flags cut to the core of people's identity and their belief system," claimed Democratic Unionist councillor Christopher Stalford.
He contended that most of the community wanted the British flag to stay.
But Sinn Fein's Gerry Kelly accused the unionists of seeking confrontation and the police of mounting an incompetent security operation.
He said: "We are very, very lucky that they didn't get into the building or we could have been dealing with a lot more injuries."
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