Police say four officers were injured by missiles and petrol bombs and other hand-thrown weapons in two locations north of Belfast Friday evening.

Officers used water cannons and at least five plastic bullets to deflect the rioters.

Hundreds of protesters shut down at least a dozen roads in and around Belfast and forced a suspension of the city's bus service. A major highway in the city was also closed after a small bomb was found.

At a protest in Carrickfergus, 15 km (10 miles) northeast of Belfast, police were hit with bricks and debris and responded by firing five rounds of plastic bullets, police said in a statement.

The protests first began after Catholic council members voted on Dec. 3 to remove the British flag from city hall except for 18 official days per year.

Sectarian unrest continues to sporadically flare up in Northern Ireland, where Protestant loyalists want their country to remain a part of the United Kingdom while Catholics want unity with the Republic of Ireland.

hc/av (Reuters, AP)