( CNN ) -- Police in Northern Ireland appealed for the community 's help after a fourth night of Catholic-Protestant violence in Belfast , which saw a number of officers injured . Five people , all in their teens or early 20s , were arrested in connection with the unrest , police said . Wednesday night 's violence began shortly before 8 p.m. ( 3 p.m . ET ) and lasted for about six hours , the Police Service of Northern Ireland ( PSNI ) said . Are you there ? Share your stories , photos and videos . In one incident , a car was stolen as the driver was getting inside and the vehicle was then set on fire , police said . Assistant Chief Constable Duncan McCausland said Wednesday 's violence involved a smaller group of people than on previous nights and included a `` hardcore group '' of about 12 people . Northern Ireland 's marching season begins . They threw objects including petrol bombs and fireworks at police . Police fired back with stun grenades and water cannons , they said . `` A number '' of police officers were hurt but their injuries were not believed to be serious , the PSNI said . The violence began over the weekend after a decision to allow a Protestant parade to pass through a mainly Catholic neighborhood on July 12 , a day when Protestants march to celebrate the victory of England 's King William III over his ousted Catholic predecessor , James II , in 1689 . Known as The Twelfth , the holiday has previously been marred by violence and has been a source of tension between Catholics and Protestants for years . Police had to remove demonstrators who staged a sit-down protest to block this year 's march . Rioting erupted a short time later and more than 50 officers were hurt . The previous night , 27 officers were hurt , including three who were shot at close range by a masked man armed with a shotgun . McCausland said community leaders helped with the situation Wednesday night , but he urged them to do even more . `` A number of community workers worked hard to exercise their influence last night and I would thank them for that , '' McCausland said . `` However , I would ask them to redouble their efforts . In this way , working together , we can help bring these incidents to an end . '' British Prime Minister David Cameron on Wednesday denounced the `` completely unacceptable behavior '' of those involved in the unrest . He dismissed any sectarian cause behind the violence , pointing out Northern Ireland 's police force is under local control , no longer governed by London . `` There is no excuse for anyone not to cooperate with that police force , '' Cameron said .
Five people were arrested after a fourth night of violence in Belfast . Those arrested are all in their teens or early 20s . Protesters threw petrol bombs and fireworks at police . Police are appealing for more community help to deal with the unrest .