( CNN ) -- Day-long clashes between police and rioters marked a controversial holiday march in Northern Ireland , leaving dozens of officers injured and widespread damage in Belfast . The violence began overnight , on the eve of July 12 , known as `` The Twelfth '' by Protestants who march to celebrate the victory of English king William III over his ousted Catholic predecessor James II in 1689 . More than 100 riot police removed Catholic protesters from a road in north Belfast to allow the march to take place , journalist Peter Taggart told CNN Monday . Rioters pelted police with Molotov cocktails and other explosives , and officers fired baton rounds and used water cannon in attempts to break up the crowds , police said . A policewoman was struck in the head by an object thrown at her , and ambulance crews who rushed to the scene were confronted by rioters , police said . Her condition was unknown . Several vehicles were carjacked and set afire , police said . Police have not yet released details of the number of arrests made , or the number of people injured . However , authorities earlier in the day reported that at least 27 police officers were injured in the clashes . The injuries are not thought to be serious , police spokesman John Anderson had said . The violence took place in two different parts of the city on the eve of a holiday often marked by tension between Catholics and Protestants . Tensions in Northern Ireland as marching season begins . The disruption began minutes before the start of the holiday -- also known as `` Orangeman 's Day . '' Because of the sectarian nature of the holiday , celebrations have sometimes been marred by violence in the past . `` This is utterly wrong and I condemn it in the strongest possible terms , '' police district Chief Superintendent Mark Hamilton said in a statement early Monday . `` Those involved in tonight 's violence were intent on causing mayhem and destruction . '' Hamilton said police were investigating the incident . `` We have appealed for calm in the run up to the Twelfth of July and we continue to do so , '' he said . `` We would appeal to anyone with influence in the community to exert it to ensure that the next few days pass off without incident . '' On Saturday , police in Northern Ireland said a roadside bomb that exploded in a border caused significant damage to both a road and a nearby bridge . District Cmdr . Chief Alasdair Robinson said he believed that police officers were the target , and that the bomb was placed in the area in an attempt to injure or kill them .
Rioters attack police on historic Protestant cemetery . At least four police injured in two different locations , including three hit by shotgun blast . Police throw stones and explosives . Disruption occurs on eve of holiday .