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.