Northern Ireland riots spread

Cars were ignited during the second night of violence

BELFAST, Northern Ireland (CNN) -- Riots flared for a second night in Belfast, with cars being set on fire and police and army under heavy attack.

Rocks, petrol bombs and explosives were thrown at the security forces as they kept rival gangs of Protestant and Catholic youths apart.

At least 20 police officers were hurt in the violence, described as less intense than Wednesday disturbances, but trouble did spread to other parts of the city.

A Catholic school close to an intersection of the two communities was burned despite earlier efforts of community leaders on both sides to reduce tensions in the city.

AUDIO CNN's Nic Robertson: Tension in Belfast 1. 35 MB / 2 mins, 8 sec AIFF or WAV sound

As the attacks intensified six blast bombs were hurled and police said 10 shots were fired at officers, the UK's Press Association reported.

Three people were arrested for public order offences.

About 24 petrol bombs were recovered by the security forces in the Ardoyne area. Cars were set on fire in the Ardoyne before the area was declared quiet at 4.20am.

More Royal Ulster Constabulary officers were injured as trouble spread out of the north of the city into the west.

Tensions are particularly high because the height of the Protestant Orange Order marching season is approaching, CNN's Nic Robertson said.

The outbreaks of trouble in the city have been characterised by their proximity to routes of contentious marches.

Close to one route Protestant pensioners say their houses have been attacked.

Despite the calls for calm, security forces are still on high alert.

Rioting has always been a regular feature of Northern Ireland's cross-community hatred and it is rarely more volatile than when the political stakes are as high as they are now, Robertson said.

High level political talks are expected to resume on Friday aimed at keeping Northern Ireland's fragile peace process on track.

Workers began cleaning up after Thursday's rioting

The Catholics say Protestants began throwing rocks on Tuesday at schoolgirls leaving Holy Cross Primary School, which sits beside the few remaining Protestant-occupied streets in Ardoyne, Associated Press reported.

"This trouble will stop when Protestant community leaders can say with confidence that these children can go to their school in safety," said Gerry Kelly, a former Irish Republican Army bomber who today is a north Belfast representative of the republican Sinn Fein party.

The Protestants -- who on Thursday blocked the main road outside the school and forced parents to escort their children through a back entrance -- insist the violence began on Tuesday when IRA supporters from Ardoyne attacked Protestants erecting flags on Protestant streets beside the school.

Thirty-nine police officers were injured and five treated in hospital following the rioting on Wednesday, the RUC said.

"I think this is some of the most serious rioting that Belfast has seen for several years. We have had crowds of up to 600 involved at different times," said RUC Assistant Chief Constable Alan McQuillan.