In this story:

• 'We owe these youngsters more'
• Trimble appeals to 'brethren'
• Clinton 'deeply saddened' by attack
• First deaths since Orange parade ban
• Catholic funeral mass planned
• Related sites and stories

BELFAST, Northern Ireland (CNN) -- The Protestant Orange Order said Sunday they would continue the standoff outside Portadown "indefinitely" and prepared to march on their biggest holiday despite calls to end their protest after three young brothers were killed in an arson attack.

The deadly bombing early Sunday in Ballymoney, northeast of Belfast, has significantly raised tension in the British province. Political leaders on both sides called for the Orange Order to end its protests over a banned parade in Portadown.

The 12th of July is the Orange Order's biggest holiday, commemorating the defeat of Catholic King James II by King William of Orange at the Battle of the Boyne on July 12, 1690.

Although the battle's anniversary was on Sunday, the observances are being held Monday because Protestants prefer not to march on Sunday.

"After last night's attack a 15-minute walk down Garvaghy Road by the Orange Order would be a very hollow victory because it would be in the shadow of the coffins of three little boys who wouldn't even know what the Orange Order is about," said the Rev. William Bingham, chaplain to the Orange Order in County Armagh.

Protestant assembly leader David Trimble and his Catholic deputy Seamus Mallon urged the Orange Order in a joint appeal to call off protests at Portadown's Drumcree church, where they are demanding to be allowed to march down the mainly Catholic Garvaghy Road.

"We appeal to the Orangemen at Drumcree to immediately end their protest and return to their homes," the two leaders said.

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A hollow-eyed Ronnie Flanagan, head of the Royal Ulster Constabulary, said he considered the boys' deaths a "turning point" in the increasingly fractious standoff over the Orange Order's stymied attempts in the past week to march down Garvaghy Road.

"These are the real victims of our troubles, murdered while they slept in their beds," Flanagan said. "For me, this changes everything. There are things clearly much, much more important than marches or opposing marches."

"We owe these youngsters more. We owe all our youngsters more."

Trimble appeals to 'brethren'

Trimble, the local member of Parliament and himself a member of the Orange Order, addressed what he called the "brethren" in the wake of what he described as an appalling act of barbarity.

"I must say to the Portadown brethren that the only way in which they can clearly distance themselves from these murders, show to the world that they repudiate those who have murdered young children ... is now to leave the hill at Drummer Parish Church and return home," Trimble said.

But most Orange Order members disavowed responsibility for the attack.

"All the citizens of Northern Ireland are responsible because of the situation we have found ourselves in over the years," Orange Order spokesman David Jones said.

The Rev. Ian Paisley, the uncompromising hard-line Protestant leader, spoke with brimming emotion of the "diabolical, hellish deed" that took the boys' lives.

Earlier in the day, British Prime Minister Tony Blair denounced the arson attackers as "vile and vicious sectarian murderers" -- a condemnation that was shared by Paisley, a pro-British politician-cleric who is the member of Parliament for Ballymoney, where the bombing took place.

The White House expressed hope that the brothers' deaths would underscore the human costs of confrontation and violence.

Describing President Clinton as "deeply saddened" by the attack, White House press secretary Mike McCurry said, "The promise of peace is best achieved through dialogue and reconciliation."

Mo Mowlam, Britain's Northern Ireland secretary, urged "mothers, fathers, sons and daughters" to stay home Monday, one of Northern Ireland's biggest Protestant marching days, for the sake of the province's fragile peace. All Protestants and Catholics should "think twice before they exit their front doors," she said.

Belfast was a ghost town as armored vehicles moved in Sunday in anticipation of the Orangemen's Lower Ormeau Road parade, second only to Portadown in its capacity to fuel sectarian emotions.

The three deaths at Ballymoney were the first in the unrest that has gripped Northern Ireland since the Orange Order was banned July 5 from staging its annual parade along Garvaghy Road.

"I can now confirm that we are treating this as a brutal murder of three young Catholic boys in the Carnany [housing] estate in a sectarian arson attack," police chief inspector Terry Shevlin said.

The brothers were members of the Quinn family -- Richard, 10; Martin, 9; and Jason, 7.

Their Catholic mother escaped with her Protestant boyfriend, who was beaten back by flames when he returned to try to rescue the brothers, police said. There were reports the boys were being raised as Protestants.

A photograph released by the family showed three impish little boys in sweatshirts, Richard's grin revealing a wide gap in the middle of his teeth.

Chrissy Quinn, the boys' mother, and boyfriend Raymond Craig had lived in the housing estate for only one week. Francis Quinn, the sister of the boys' mother, said the family had been harassed for two years.

"They murdered three innocent little children. They weren't being brought up Catholic or Protestant -- they were just three kids," Frances Quinn said.

A Catholic funeral Mass for the boys was planned for Tuesday morning, said parish priest Peter Forde.

Forde said he twice visited the boys' mother, who was treated for minor injuries from the fire and found her in a state of deep shock.

"She discharged herself," he said. "The hospital didn't want her to leave, but that wasn't their choice.

Northern Ireland's independent parades commission Sunday rejected a fresh application from the Orange Order to stage its annual parade down Garvaghy Road.

A statement from the commission said the Orange parade was still "prohibited," in line with its original ban.

Thousands of Orangemen have remained encamped outside barricades in Portadown, staging increasingly bold attacks on police once night falls.