Police in Northern Ireland fired water cannon and at least one plastic bullet round at rioters who had attacked them with bricks and bottles in the latest outbreak of anger at the removal of the British flag from Belfast City Hall, Reuters reported.

Hundreds of other protesters brought large areas of Belfast to a standstill, shutting at least a dozen roads and forcing the shut-down of the city's bus service.

The unrest over the past five weeks has been some of the most sustained in the British-controlled province since a 1998 peace deal ended 30 years of conflict between Catholic Irish nationalists seeking union with Ireland and Protestant loyalists determined to remain part of the United Kingdom.

Loyalists have held nightly protests since nationalist councilors voted last month to end a century-old tradition of flying the British union flag every day over the city hall, exposing a deep vein of discontent with the peace deal.

Loyalist politicians have joined their nationalist rivals in condemning the violence, but they have been unable to prevent groups of young men draped in British flags from clashing with police.

At least 200 youths, many covering their faces with scarves and hoods, gathered near a roundabout near Newtownabbey in north Belfast on Friday and some set fire to a double-decker bus.

Within an hour several dozen were firing petrol bombs, paint bombs and fireworks at police, who responded with water cannon. Hundreds of mainly teenage protesters looked on.

At a separate protest in Carrickfergus, 15 km (10 miles) northeast of Belfast, police were pelted with bricks and bottles and responded with one plastic bullet round, police said in a statement.

Police regained control of the area by blocking a number of protesters into a side street and making several arrests as hundreds of locals looked on.