Even hard-line Protestant politicians expressed revulsion at the attack, which caused some 100 students and their parents to flee in panic. The incident followed two days of taunting and violent harassment of the schoolgirls and two nights of rioting that have left the Ardoyne neighborhood littered with burned-out cars and rubble.
Two soldiers and at least 45 police officers have been injured, and more than 250 bombs were thrown, according to police reports.
In a separate incident in Belfast Tuesday night, a 16-year-old Protestant died after his bicycle was rammed by a car.
The Red Hand Defenders, a cover name used by Protestant paramilitary organizations, claimed responsibility for today's bomb attack. The police arrested three men in connection with the bombing.
Billy Hutchinson, a hard-line Unionist politician with ties to Protestant paramilitary groups, could hardly hide his disgust. "There is nothing that justifies this," he said, pointing in the direction of the spot where a police officer lay in the street after the explosion.
"I want to walk away from it, and even leave this country," said Mr. Hutchinson, whose Unionist, or loyalist, camp wants Northern Ireland to remain part of the United Kingdom. "It isn't worth fighting for any longer. This is not about children going to school any longer; this is about people hellbent on causing trouble. I'm sick to my stomach, and I'm ashamed to be a loyalist."
Dozens of armored vehicles lined Ardoyne's main street today to create a corridor for the students. With machine guns at the ready, British Army soldiers stood near Protestant homes to prevent residents from clashing with parents.
In one sense, the schoolgirls have become pawns in a long-running struggle for supremacy over the rough concrete streets of North Belfast. Reforms ushered in with the 1998 Good Friday peace agreement have given Catholics more political power, and the Catholic population in this part of the city is growing, making Protestants feel under siege.
On Monday and Tuesday, a cordon of officers in body armor walked in front of the students — some as young as 4 — and drew nightsticks as they chased Protestants back through the front yards of private houses.
Today, fewer protesters were on their doorsteps, and fewer epithets were shouted. Then a shower of stones and broken bricks slammed into the police jeeps and into the street, and a thunderous bang came seconds later. Schoolgirls ran screaming.
"I feel terrified," said 6-year-old Aine Booth as she stood at the school entrance with her mother after the bomb went off.
Protestants began blockading the street last June to protest alleged harassment by Catholic residents, and they successfully forced the Holy Cross Girls' Primary School to start its summer vacation early. The Catholic school is a few hundred yards from a "peace line," a cross- street separating Protestant and Catholic homes, but on the Protestant side.
Many parents have kept their daughters away from school. Only half the 230 pupils have attended this week, according to the principal of the school, Anne Tanney, and half those who do come have been too upset to stay.
"They were very traumatized," Ms. Tanney said. "We couldn't teach them under those conditions."
The violence in Ardoyne has followed weeks of turmoil in Ulster politics. Britain was forced to suspend home rule after David Trimble, the leader of Ulster's Unionists and first minister in the power-sharing government, resigned on July 1 to protest the Irish Republican Army's failure to disarm. Sinn Fein, the I.R.A.'s political wing, has said that it is impossible unless Britain reduces its military presence in Ulster and the police force is reorganized to include many more Catholic officers.
Then, last month, three members of the I.R.A. were arrested in Colombia on suspicion of training its leftist rebels in guerrilla warfare. Roman Catholic politicians have denounced the blockade along the Ardoyne road. "It's clearly anti-Catholic sectarian bigotry," said Martin Morgan, a local legislator for the Social Democratic and Labor Party, a moderate Catholic political party.
Secretary of State John Reid, Britain's representative in the province, returned early from his vacation.
The police have advised parents to take their daughters to the Holy Cross School by a circuitous alternative route, and most of those who have gone to class have heeded that advice. But the police have also pledged to protect the direct route along the Ardoyne Road.
"The road must be used, " said the Rev. Aidan Troy, a member of the Holy Cross Girls' Primary School's governing board, "because otherwise we have decided to retreat into barbarity."
Community leaders pledged to reopen a dialogue between Protestant and Catholic residents, but few were hopeful. "We're all to blame," Ms. Tanney said. "This should never happen to children."