Five police officers were injured as loyalists rioted in Belfast after a controversial vote on the Union Jack flag.
The violence spread to parts of east Belfast, 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 the hundreds of loyalists throwing missiles.
Five police officers were injured during the disturbances.
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.