Police in Northern Ireland fired plastic bullets and water cannon at rioters who wounded four officers with missiles and petrol bombs in the latest outbreak of anger at the removal of the British flag from Belfast City Hall.

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. A major highway in the city was closed after a small bomb was found.

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 UK.

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

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 in Newtownabbey, North Belfast and some set fire to a double-decker bus.

Within an hour several dozen were launching 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 northeast of Belfast, police were pelted with bricks and bottles and responded with five rounds of plastic bullets, police said in a statement. Police regained control of the area by blocking a number of protesters into a side street and detaining several.

Many roads in central Belfast were deserted early in the evening after protest organisers promised on Twitter to shut down the city. Police reported that at least 12 roads had been closed, although some were later reopened. The city’s main bus routes were suspended. The Westlink dual carriageway, a Belfast traffic artery, was closed while police removed a “small viable pipe bomb,” police said.

The protesters have complained that the removal of the flag was a step too far in the ebbing of loyalist dominance in the province, saying too many concessions had been given to Irish nationalists in a power-sharing government.

But exasperated residents from both communities, concerned at the impact of the protests on the economy and the city’s image, have called on the protesters to halt the rioting and address their concerns to politicians.