Gangs of young men threw gasoline bombs and bricks at policemen and set cars and a city bus ablaze in a third night of violence in Protestant districts of Belfast that has injured more than 50 police officers and at least 10 civilians.

The city's worst rioting in years was touched off Saturday when police refused to allow the Orange Order, a legal Protestant fraternal group, to parade through a largely Catholic neighborhood.

Anger at that decision dissolved into brawling and burning, in which largely Protestant gangs ripped down streetlights with a hijacked backhoe, torched cars and tossed firebombs as more than 1,000 police officers tried to restore order.

The violence broke out at a time when a disarmament pledge by the Irish Republican Army in July had lifted hopes of long-term peace in this British province. Protestants said the rioters were angry over what they have called appeasement of the anti-British group, which has yet to announce destruction of any weapons.

Shots were fired at police in a few areas on Saturday and Sunday, and the violence spread beyond Belfast to several outlying towns. The injured Sunday included a 20-month-old boy in Belfast who suffered a fractured skull when gangs beset his family's car, and a woman in her seventies who was struck by rocks thrown by men in Bangor, east of Belfast.

Heavily armored police trucks were parked near entrances to Catholic neighborhoods, where gangs of young men stared down the darkened streets toward the Protestant areas for signs of trouble coming their way.

On Monday afternoon, Protestant crowds conducted nonviolent demonstrations, blocking traffic at major intersections. Things turned violent late Monday night. A city bus burned on Cambrai Street in north Belfast, an area where streets were eerily dark and menacing gangs of youths -- including some who appeared to be younger than 10 -- stood on street corners amid broken glass and smashed guardrails.

Later Monday evening, police said gangs on the street threw rocks and firebombs at police officers who responded to the scene of the burning bus. One officer was hospitalized after being hit in the head with a brick, said police spokesman John O'Rourke.

More than 3,600 people have died in three decades of what people here call "the Troubles," an economic, religious and political struggle between largely Protestant "loyalist" groups fiercely faithful to the British crown, and largely Catholic "republican" groups who oppose British rule and believe the province should be united with the Republic of Ireland.

The IRA has long been the most visible symbol of the violent struggle, but this summer the mostly Catholic militia group announced it was laying down its weapons. Britain responded by withdrawing troops and dismantling military posts while furious loyalist groups said London was abandoning them and leaving them unprotected. Many Protestant groups have felt betrayed since the Good Friday peace accords of 1998, which established Catholic and Protestant power-sharing in the regional assembly.

Protestants interviewed on radio here Monday said the British government had rewarded the IRA's long campaign of violence by agreeing to phase out troops. They said loyalists were trying to force Prime Minister Tony Blair's government to listen to their demands.

Officials have said it is unclear whether the violence has been orchestrated by loyalist paramilitary groups, such as the outlawed Ulster Volunteer Force and the Ulster Defense Association.

Standing in front of the burning bus on Cambrai Street, one resident shook his head in disgust. "Don't call these people loyalists," he said. "I'm a loyalist. A loyalist obeys the laws of the land. A loyalist is a Protestant, and Protestants are believers in a reformed faith. These people are thugs."

After meeting with the province's top police officer, Peter Hain, the British government official in charge of North Ireland affairs, said Northern Ireland's Protestants faced a "moment of choice" between "law and order, fairly applied" and armed attacks on police.

David Ervine, leader of the Progressive Unionist Party, which is closely linked to the Ulster Volunteer Force, said the violence resulted from "a perception that this community is being set aside and ignored. . . . If there's anything to be gained from this, it's the hope that people will now pay attention to these concerns."

Special correspondent Mary Fitzgerald contributed to this report.