Belfast , Northern Ireland ( CNN ) -- 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 . Read more : Clinton urges calm amid tensions in Northern Ireland . 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 December 3 decision to limit the flying of the flag to 18 days per year . The disorder has resulted in more than 100 arrests and left 66 officers injured . The flag was raised Wednesday for the first time since the controversial vote to mark the birthday of Prince William 's wife , Catherine . It was lowered at the end of the day . The leaders of the two main unionist political parties met Thursday to discuss concerns within the unionist community . Read more : Political forum aims to stem N.Ireland violence . Northern Ireland authorities have accused loyalist extremists of exploiting the decision by Belfast officials to end the century-old tradition of flying the Union flag over City Hall 365 days a year . The British flag has long been a flashpoint between British loyalists and Irish nationalists , who want Northern Ireland to join the Irish republic . The Belfast City Council vote followed a summer of heightened tensions between Northern Ireland 's Catholic and Protestant communities . Riots in September left dozens of police injured . The majority of Ireland gained independence in 1921 following two years of conflict . But six of the nine counties of the province of Ulster chose to stay in the United Kingdom , eventually becoming Northern Ireland . Police : Extremist group 'orchestrating violence ' in Belfast over Union Jack . In the late 1960s , the conflict between mainly Protestant loyalists , who want Northern Ireland to remain part of the United Kingdom , and largely Roman Catholic nationalists , who want it to be reunited with the rest of Ireland , exploded into a political and sectarian war , known as `` the Troubles . '' The three decades of ensuing violence between loyalists and the IRA claimed more than 3,000 lives , most of them north of the border . While the Good Friday Agreement of 1998 , also known as the Belfast Agreement , effectively ended the conflict , distrust remains between Catholics and Protestants . Under the terms of the accord , groups on both sides dumped their weapons , and members of Sinn Fein , the political affiliate of the IRA , now work with pro-British politicians in Northern Ireland 's power-sharing government . CNN 's Laura Smith-Spark wrote in London and Nic Robertson and Peter Taggart reported in Belfast .
NEW : Four police officers are hurt , one is hospitalized . NEW : Two people are arrested . Protesters angry over a decision to stop flying the Union flag year-round . Police say 66 officers injured , 100 arrests made since December .