Police officers have come under attack in east Belfast following disorder in the Castlereagh Street and Templemore Avenue areas.
The PSNI said officers suffered sustained attack from crowds of up to 300 people using fireworks, bricks and petrol bombs. Water cannon were deployed to the scene.
A number of police officers have been injured in the disorder and several arrests have been made.
Yesterday, ten officers were injured when they were attacked by a mob of protesters in Belfast.
Two people were charged with rioting after the violence in east Belfast.
Rioting began a month ago after a vote by mostly nationalist councillors to end the century-old tradition of flying the union flag from Belfast City Hall every day unleashed the most sustained period of violence in the city for years.
First Minister Peter Robinson called the decision to take down the flag "ill-considered and provocative" but said the attacks must end.
"Those responsible are doing a grave disservice to the cause they claim to espouse and are playing into the hands of those dissident groups who would seek to exploit every opportunity to further their terror aims,” he said.
Earlier SDLP spokesperson on policing and MLA Conall McDevitt called for police in Northern Ireland to be firmer in their dealings will illegal protests.
Speaking on RTÉ's News at One, Mr McDevitt also accused unionist politicians of doing a "u-turn" on the issue of flying the union flag, and questioned whether they were stoking political tension for political purposes.
Union flag protest planned for Dublin
Loyalist activist Willie Frazer has said he is organising a demonstration in Dublin next weekend as part of the ongoing protests over the union flag controversy.
Mr Frazer, who is based in Markethill, Co Armagh, said he expects 150 people to take part in a short protest at Leinster House at midday on Saturday, 12 January.
He said the protest will be peaceful and has informed gardaí of his plans.
Mr Frazer was involved in organising an Orange Order parade in Dublin in 2006, which ended in a riot.