Belfast , Northern Ireland ( CNN ) -- Violence continued early Friday in mainly Catholic districts of Northern Ireland after protests began on Thursday , the main day of the annual Protestant marching season . The Police Service of Northern Ireland said `` at least 10 gunshots '' were fired at police lines in the nationalist Brompton Park area of Belfast 's Ardoyne district at 12.30 a.m. Friday . Police said no-one had been wounded . Officers were also attacked with pipe bombs during the disorder , police said . Police said they were working with community representatives to restore order . Problems began Thursday in Ardoyne after a parade by the Orange Order . The march passed without a major incident , but there was tension between pro-Irish and pro-British groups as they staged rival demonstrations . Both sides hurled stones and other objects , police said . Officers were wounded in an attack from pro-Irish youths throwing petrol bombs , fireworks and bricks . The Police Service said the officers ' injuries were `` not life-threatening . '' More than 20 officers were injured , although none very seriously , police said . `` A large number of petrol bombs have been thrown at police . Two arrests have been made at this stage , although this number is expected to rise in the coming days as police continue with their evidence gathering operation , '' a police statement said . The police also said several vehicles were set on fire and pushed at police lines . Riot police used water cannon and plastic bullets to bring the crowd under control . Sporadic violence continued for several hours . Police also reported minor clashes in a Protestant district beside Catholic Ardoyne . British loyalists there threw bricks and bottles at police . Assistant Chief Constable Will Kerr , the officer in charge of the policing operation in Belfast , condemned the attacks on police . `` I am angry we have this annual madness when it seems that people think the peacekeepers are a legitimate target , '' he said Friday . He added that a `` significant number '' of arrests would be made to ensure those involved in the violence `` are placed before the courts and answer for those actions . '' In a statement Thursday he said police were `` dealing with significant disorder '' and urged calm in the area . There was also disorder in the Northern Ireland city of Londonderry , or Derry , as many residents call it . Police said petrol bombs were thrown at officers in the Bogside area , the scene of the Bloody Sunday killings by British soldiers in 1972 . Vehicles were also set alight in Derry , and a public bus was hijacked and burned in Craigavon , County Armagh . Six men aged 18 to 34 have been charged with public order offenses during disturbances in Belfast on Wednesday night .
NEW : More than 20 officers hurt , two were killed after an Orange Order parade in Belfast . NEW : ` ` I am angry we have this annual madness , '' a police chief in Belfast . Pro - Irish and pro - British groups threw objects as they staged rival demonstrations , police say . British soldiers and police clash in a Protestant district beside Catholic Ardoyne .