The vote late last year to end a century-old tradition of flying the flag has sparked some of the most sustained street violence in the British-controlled province since a 1998 peace deal ended 30 years of tit-for-tat killings.
Loyalist politicians, who want to maintain a union with Britain, have joined their Nationalist rivals, who want a united Ireland, in condemning the violence.
But they have been unable to prevent groups of young men draped in British flags from clashing with police on an almost nightly basis.
Police said at least eight roads around the city had been closed by protesters. Over 100 gathered at the Rathcoole estate in north Belfast and set fire to a bus, a Reuters witness said.
Television footage showed dozens of protesters at two other points in the city and at least 200 protesters gathered in the nearby town of Glengormley.