Police in Northern Ireland fired plastic bullets and water cannon on Friday as pro-British loyalists furious over restrictions on flying the British flag torched a bus and hurled petrol bombs at officers.

Violence also flared in towns outside the capital Belfast as loyalists -- the Protestant community's working-class hardcore -- blocked roads around the province to express their anger.

Northern Ireland has been swept with a wave of sometimes violent protests since December 3, when Belfast City Council voted to restrict the number of days the British flag is flown at City Hall to 18 per year.

Most of Friday's province-wide protests were peaceful, with demonstrators taking the flag onto the streets, but serious disorder broke out in the towns of Newtownabbey and Carrickfergus, just north of Belfast.

Police fired water cannon and five plastic bullets at rioters after they were attacked with a total of 33 petrol bombs, as well as fireworks and masonry.

Four officers were injured, with one requiring hospital treatment, police said, bringing the total number of officers injured since December to around 70.

Two early arrests were made.

Loyalists have taken to the streets most nights since the Belfast City Hall ruling.

They see the council's decision to restrict the flying of the flag as an attack on their identity and an unacceptable concession to republicans seeking a united Ireland.

In the seaside town of Carrickfergus on Friday, dozens of armoured police vehicles drove in to restore order after around 100 protesters threw bricks, bottles and fireworks and torched plastic trash bins.

In the shadow of Carrickfergus Castle, rubble littered the junction of Irish Gate and West Street. Groups of youths in tracksuits lingered as police in riot gear with plastic shields stood by.

One Carrickfergus man who did not want to be identified blamed the disorder on youths from Belfast who had travelled north on the train.

"I could see them from my house coming out of the station, young guys aged 15 or 16," he said.

Some youths kept warm standing by burning debris.

Queen Elizabeth II gave her grandson Prince William the subsidiary title Baron Carrickfergus when he married Kate Middleton in 2011.

In nearby Newtownabbey, a double-decker bus was set alight on the Rathcoole housing estate, sending clouds of black smoke into the rainy skies.

Masked youths later threw petrol bombs and missiles when police entered the estate.

The Westlink, which joins Northern Ireland's M1, M2 and M3 motorways in Belfast, was closed for three hours following a bomb scare.

Army technical officers found a "small, viable pipe bomb-type device" which was removed for further examination, a police spokesman said.

A car was also torched in Belfast city centre.

A 1998 peace agreement brought an end to the three decades of sectarian violence between Protestants and Catholics known as the Troubles, but sporadic bomb threats and murders by dissident republicans continue.