Five police officers have been hurt during disorder in Belfast that erupted minutes after the council had voted to change its policy on the Union flag.
A number of loyalist protesters tried to force their way into Belfast City Hall where the vote took place.
Two security staff at the hall and a press photographer were also injured. Separately, trouble broke out on the Lower Newtownards Road in east Belfast.
The council voted to fly the flag only on 15 designated days during the year.
It marks a change to the current policy by which the flag is displayed outside the building 365 days a year. The council's decision has angered many loyalists.
Some of the protesters who had gathered outside the city hall used metal barriers, bottles and golf balls to attack police.
A PSNI spokeswoman said they had deployed additional resources to deal with public disorder in both central and east Belfast.
There was an attempt to hijack a bus on the Albertbridge Road, and there are reports that St Matthew's Catholic Church came under attack.
Of the five officers who were hurt, two women were taken to hospital with injuries that are not believed to be life threatening.
An Associated Press photographer sustained a head injury while covering the violence at the city hall.
The vote was called when nationalist councillors - who now hold a majority on the council - proposed a motion to remove the Union flag completely.
However, both Sinn Fein and the SDLP backed a compromise amendment, proposed by the Alliance party, to display the flag on designated days, in line with the current policy at Stormont.
The Alliance motion was passed by 29 votes to 21.
Minutes later, a number of loyalist protesters broke through the rear gates of the grounds of the city hall and tried to force open the doors of the building.
BBC Northern Ireland's political reporter, Martina Purdy, who was at the scene, said one security guard appeared to be bleeding from a head wound while a policewoman had sustained a hand injury.
She said the protesters had shouted "shame" and "no surrender" when they broke into the courtyard, waving flags.
A pane of glass was shattered as the demonstrators tried to break open the doors of the 106-year-old building.
Windows were also smashed on a number of cars parked within the courtyard, including a vehicle belonging to a DUP councillor.
SDLP councillor, Tim Attwood, who took part in the vote, said: "This was an appalling spectacle, resulting in significant damage to property and, most alarmingly, injury to a number of those seeking to keep city hall secure, and our thoughts are with those who were hurt.
"Any attempt at a resort to mob rule cannot be countenanced."
DUP councillor, Christopher Stalford, said there was "absolutely no excuse" for the violence which erupted after the vote.
However, he added: "Those who started this debate should have known from the outset that it would stir up tension and cause division.
"I trust as they look back on the damaged relationships, costly consultations and energy expended, they will realise their focus was wrong.
"Indeed, all those who supported this stripping away of British identity are just as guilty and foolish."
Marie Hendron from Alliance, said she believed the violence had been "orchestrated". She said it was a "disaster for this city".
However, she said it was also a "historic night for Belfast" and the council had shown its commitment to a shared future.
"For the first time in their history both Sinn Fein and the SDLP have voted in support of the Union flag. This proves in a practical way that they acknowledge the constitutional position of Northern Ireland," the Alliance councillor added.
Ahead of the vote, more than 1,000 loyalists gathered in May Street to protest about the proposal to change the flag policy.
Nationalist councillors had argued that removing the flag would create a more equal and neutral environment in a divided city.
However, unionist councillors had accused Sinn Fein and the SDLP of "raising tensions" and abusing their majority on the council.
Combined, Sinn Fein and the SDLP now outnumber unionist councillors by 24 to 21.