Will Kerr, assistant chief constable of the Police Service of Northern Ireland, said some of the violence was orchestrated by paramilitary groups, the Belfast Telegraph reported. At least eight police officers were injured in scuffles with demonstrators.
The Belfast City Council voted Monday to stop flying the flag 365 days a year, limiting its display to 17 days. The new rules match those adopted for Stormont Castle, the seat of government in northern Ireland.
A demonstration Monday night turned into a riot, spreading into East Belfast. Since then, two bombs have been found and a member of Parliament has received a death threat.
Kerr urged everyone to get on with Christmas shopping and warned that police would pursue violent offenders.
"I am urging everyone to be calm, take a step back and think about how this violence is affecting not just their own communities but the whole of Northern Ireland," he said.