Protests broke out Friday throughout Northern Ireland against a decision by Belfast City Council members to stop flying the Union flag year-round. Most of the protests were peaceful, although disruption was caused to the road network in Belfast and across the country, police said in a statement. One of the most serious incidents occurred at O'Neil's Road in Newtownabbey, where about 100 Protestant rioters -- many of them teenagers -- threw concrete blocks, paving slabs, bricks, planks of wood, gasoline bombs and Molotov cocktails at police. They targeted about 30 armored police vehicles and as many as 100 police officers clad in riot gear. At least one officer appeared injured. His head bowed, the officer was helped away by comrades. Amid rain and temperatures just above freezing, police brought in two water cannon, deploying one of them to push back the rioters. Police were trying to gain control of a main highway. In pictures: Flag protests in Northern Ireland In all, four police officers were hurt, with one requiring hospital treatment, and two people were arrested, police said. Thirty-three gasoline bombs were thrown at police during the demonstrations, after which all the roads were reopened. The eruptions occurred after two nights of calm that followed five consecutive nights of rioting. Belfast has been at the center of a wave of protests prompted by the City Council's