BELFAST -- Protestant hardliners attacked lines of Belfast riot police Friday as Northern Ireland's annual mass marches by the Orange Order brotherhood reached a furious, chaotic end with running street battles at several urban conflict zones.

In north Belfast, police girded in flame-retardant suits and helmets deployed a half-dozen armoured cars to block a road that Protestant Orangemen wanted to use to march past a hostile Catholic district. Men jumped on top of the vehicles blocking their path and, as crowds of blocked marchers built up behind them, threw bottles and bricks point-blank into police lines. Many in the mob cheered as at least one policeman, struck and knocked semiconscious, was pulled back by colleagues.

Officers responded by firing a massive mobile water cannon at the rioters, knocking at least one man off an armoured car and leaving him sideways on the pavement, his forehead bloodied. But the Protestant crowd kept swelling and launching objects into police lines, forcing officers to respond with volleys of snub-nosed plastic bullets in a failed bid to force the crowd to retreat. Amid the melee, a second injured officer crumpled to the ground.

Leaders of the Orange Order vowed to keep Protestants rallying to the confrontation zone until police caved in and permitted the march past the militant Catholic district of Ardoyne, the most bitterly disputed spot on the Belfast map. The police commander, Chief Constable Matt Baggott, said his force would stand its ground and gather video evidence against potentially hundreds of rioters.

This year, a British-appointed Parades Commission barred the Orange parade from using the main road beside Ardoyne to return Friday night to their nearby Orange lodge because, for the previous four years, Irish Republican Army die-hards based in Ardoyne had attacked police with gunfire, grenades, firebombs and other weapons and wounded more than 250 officers in the process.

Friday night's clashes at Ardoyne and several other Catholic-Protestant dividing lines in Belfast threatened to overwhelm the 4,000 Northern Irish police and 630 reinforcements imported from Britain who were trying to keep control of the streets for "the Twelfth," Northern Ireland's official sectarian holiday, when the British Protestant majority commemorates a 17th-century military victory over their Irish Catholic foes.

Police faced angry crowds in several other areas, particularly around Short Strand, the only Catholic enclave in otherwise Protestant east Belfast. There, rival crowds of youths traded salvos of bottles, bricks and golf balls over high security fences called "peace lines." Police advised motorists to avoid much of north and east Belfast to avoid becoming trapped in the mob violence.

Before the rioting began, Orange leaders marched to the Parades Commission headquarters and unfurled a banner that read, "We will not be defeated. No surrender."

While insisting they didn't want Protestants to attack police or Ardoyne Catholics, Orange leaders laid blame in advance for any bloodshed on the Parades Commission, a cross-community panel that since 1997 has restricted Orange marches from passing near Catholic communities.

"The Parades Commission are showing that violence pays. By their actions they've created this crisis," said the Rev. Mervyn Gibson, the Orange Order's chaplain and a former police officer.

When asked whether Protestants should respect the Parades Commission's legal authority, Gibson said no.

"We cannot accept something that is so deliberately stacked against us. This is not about 300 yards of road. This is people trying to get home," he said.

Catholic leaders accused Orangemen of violating a series of Parades Commission restrictions during morning parades through Belfast.

They noted that some of the order's accompanying bands of fife and drum -- called "kick the pope bands" by both sides in a candid admission of their intention to irritate the Irish side -- ignored orders not to play overtly sectarian tunes as they passed an inner-city Catholic church, a venue for previous street fights.

Bands similarly ignored those instructions as they passed the church again in the evening. Accompanying crowds chanted anti-Catholic songs.

And Protestant supporters of the marchers temporarily overwhelmed police units at the area, sprinting around them as flak-jacketed officers on foot shouted "Cordon! Cordon!" in a failed bid to keep the Protestants from running into nearby Catholic areas and throwing rocks at the windows of homes.

"The Twelfth" commemorates the July 12, 1690, triumph of Protestant King William of Orange against the Catholic he dethroned, James II, in the Battle of the Boyne south of Belfast.

The Orange Order, founded in 1795 as a force for uniting disparate Protestant denominations under one anti-Catholic banner, was instrumental in creating Northern Ireland in 1921 shortly before the predominantly Catholic rest of Ireland won independence from Britain.

Catholic clashes with police over Protestant marches triggered the rise of Northern Ireland's modern conflict in 1969. The issue has defied resolution despite a two-decade peace process that has delivered paramilitary cease-fires, British military withdrawals, police reform and a stable Catholic-Protestant government.

Friday's parades attracted unusually heavy crowds, reflecting exceptional sunshine on what was the hottest day of the year. Among the Belfast spectators, many bedecked in Union Jack-patterned hats and sunglasses, was a sleeping infant bearing a bib that read, "My 1st Twelfth."