Political leaders appealed for calm in east Belfast after violence flared with petrol bombs and other missiles thrown.
Sinn Fein blamed scores of masked men, who they said were wearing camouflage clothing and surgical gloves, for launching coordinated attacks on the republican Short Strand area.
Ulster Unionist Michael Copeland said he believed the violence followed attacks on Protestant-owned homes, but representatives from both sides called for an end to the rioting.
Belfast mayor Niall O Donnghaile, a councillor based in the Short Strand area, said a number of nationalist residents had been injured, including one man knocked unconscious when he was hit on the head with a brick. Police were also attacked during the disturbances and advised motorists to avoid the area.
Mr O Donnghaile said: "There is no doubt that this was unprovoked and was a carefully orchestrated and planned attack on the area.
"Homes have been attacked with petrol bombs and paint bombs, bricks, golf balls. I saw what happened."
He claimed loyalist attacks had sparked violence from the republican side of the divide.
But Mr Copeland said: "I would say it was several hundred involved in very serious, almost hand-to-hand fighting. You will always get two sides to these stories. My understanding is that homes on the Newtownards road have been attacked."
He claimed the attacks had happened for several nights. Mr Copeland told the BBC the violence must stop.
"To be honest it doesn't really matter who is responsible at this stage, it's getting it stopped appears to be the problem," he said.