Police and rioters fought hours of pitched battles with firebombs and water cannon Thursday night, after a day of marches by Protestants that were denounced by Roman Catholics as a provocation.

Police said at least 55 officers were hurt in fighting that erupted when Protestant marchers from the Orange Order passed close to a volatile predominantly Catholic neighborhood in Belfast. About 200 Catholic protesters battled police in riot gear who sought to keep them away from the marchers.

Thick black smoke from half a dozen torched cars billowed into the sky, and the streets were littered with broken glass and hunks of brick. Late last month, the same area — Ardoyne, a run-down neighborhood of red-brick terrace houses on the north side of Belfast — was the scene of Northern Ireland's worst sectarian clashes in three years.

Youths, some with scarves wrapped around their faces like masks, could be seen running toward the fighting's front line, while other combatants staggered away with bloodied faces. A pair of youths ran past with a plastic container and a siphon, apparently for making firebombs. Two more moved down the street, smashing car windows.

The province's police force said Catholic protesters forced their way through a blockade set up to keep the two sides apart. Catholics said police fired on them as they tried to stage a peaceful demonstration.

Pelted with bricks and firebombs, police responded with plastic bullets and jets from water cannon.

Finger-pointing began even before the fighting died down. Nigel Dodds of the hard-line Protestant Democratic Unionist Party described the fighting as "vicious attacks" on police by Catholic rioters.

Gerry Kelly, a senior member of the Irish Republican Army-allied Sinn Fein party and a member of the provincial assembly, said the violence was "absolutely" a direct result of Orange Order parades.

"This is an anti-Catholic group, marching near Catholic areas," he said.

Elsewhere, Catholic protesters clashed with marchers in the seaside town of Ballycastle, hurling rocks at one another before stoning police who tried to separate them.

The outbreak of violence in the evening followed what had been a relatively peaceful day of parades by Protestants, who marched in hundreds of separate precessions on the biggest day of Northern Ireland's summertime "marching season." Thursday's marches commemorated a 1690 military victory by Protestant William of Orange over Catholic King James II.

The marches coincided with a push by British Prime Minister Tony Blair, his Irish counterpart Bertie Ahern and representatives from Northern Ireland's main political parties to defuse an acrimonious dispute centering on the outlawed Irish Republican Army's refusal to give up its hidden armaments. The talks, which began Monday and took a one-day hiatus Thursday, were to resume Friday.

In Washington, the U.S. State Department urged all parties to make a maximum effort to resolve outstanding disagreements over reform of the province's predominantly Protestant police force, more Brish troop pullouts and the scrapping of IRA weapons.

"We're concerned that failure to resolve these issues could contribute to heightened tensions and instability in Northern Ireland," State Department spokesman Richard Boucher said.

The marchers, drawn from the ranks of the Orange Order, Northern Ireland's largest fraternal organization, paraded to drum-and-fife music or bagpipe tunes under skies that turned from sunny to gray, clad in their traditional garb of dark suits, ties, white gloves and sashlike orange collars.

Some potential trouble was headed off earlier in the day when authorities kept marchers out of Catholic areas. In Belfast, parade participants were told to stay out of a Roman Catholic enclave off Lower Ormeau Road — an order was backed up with a tall steel barrier.

About a hundred members of the Orange Order's Ballynafeigh lodge laid their banners against the dull-green barrier and spent the next two hours praying, singing hymns and venting outrage over the parade restrictions.

From the Catholic side of the bridge, a few spectators watched, some angry and disgusted. "It's good they've got to keep away, but are we to thank them from leaving us be?" said Kathleen Doherty, pushing a baby carriage.