( 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 holiday . The violence erupted after a Catholic priest was shot dead in the city of Rio de Janeiro by two men who said they were protesting against his support for Brazil's new leftist government. People throw objects and explosives . Disruption began on eve of holiday .