Several rioters and at least nine officers were injured, none seriously, when Irish nationalists in Ardoyne, a militant Catholic enclave of north Belfast, tried to block a parade by the Orange Order, Northern Ireland's major Protestant brotherhood.
Tens of thousands of Orangemen spent Monday mounting hundreds of similar parades in an annual stress test for the province's fragile peace. Most passed peacefully, but a handful attracted violent protests that Catholic leaders blamed on Irish Republican Army dissidents opposed to Northern Ireland's joint Catholic-Protestant government.
Gerry Kelly, a minister in that 2-year-old coalition from the major Catholic-backed party Sinn Fein, said the dissidents were pursuing an "anti-peace process and sectarian agenda" that seeks to stoke tensions with the Protestant majority and torpedo power-sharing.
More than 1,000 Orangemen and their accompanying bandsmen eventually did march down the main road past Ardoyne to the beat of a lone drum - but only after riot police fought an hourlong street battle backed by a surveillance helicopter and three massive mobile water cannons.
At one point, masked Catholic rioters on store rooftops directed a deluge of Molotov cocktails, bricks and golf balls on riot police below. The officers were protected with flame-retardant suits, helmets and shields.
Later, as the water-cannon gunners sought to take rioters' legs out from under them, Catholics wearing scarves over their faces took cover behind low brick walls and post boxes. They threw rocks, bricks, bottles and even planks of wood that bounced harmlessly off the armored sides and metal-grilled windows of the water-cannon vehicles.
The Ardoyne Catholics' showdown with police continued long after the Orangemen had passed by.
Police said a gunman fired at least one live round at police lines but missed. Rioters also stole three vehicles, set them on fire - and pushed two of them toward police lines. Officers in reply fired at least 18 British-style plastic bullets. The blunt-nosed cylinders are designed to pummel rioters without penetrating flesh.
A senior Belfast policeman, Assistant Chief Constable Alistair Finlay, condemned the anti-Orange rioters as offering "the worst possible face of Northern Ireland - a face of bigotry, sectarianism and intolerance."
These were the worst riots in Belfast since 2005, when the same Protestant parade triggered much more intense and dangerous riots on the same road. Then, more than 100 police officers were wounded amid a hail of homemade grenades.
But the aftermath of that violence also illustrates how street clashes rarely rattle wider peacemaking politics in Northern Ireland. Weeks after those 2005 riots, the outlawed IRA disarmed and renounced violence, paving the way for the 2007 formation of a new Catholic-Protestant government here.
Northern Ireland's "Twelfth" holiday typically raises community tensions to their highest point of the year as British Protestants celebrate centuries-old victories over Irish Catholics.
The often elderly, conservatively dressed Orangemen are accompanied by so-called "kick the pope" bands whose hard-faced, tattooed members play an odd mix of Gospel and sectarian tunes on shrill flutes and deafening drums.
Monday's parades were preceded by a string of overnight attacks northwest of Belfast that damaged two Orange halls and two Protestant homes, one of them gutted by fire. Catholic youths cheered the blaze and jeered the home owners, a couple who vowed to leave behind their Catholic neighbors after 32 years.
Catholics daubed the Orange lodge in the village of Rasharkin with slogans praising IRA dissidents, then pelted Orangemen as they marched from the lodge. Two Protestants were hit in the head with rocks before police stepped in. Three officers were wounded as the Catholics threw several Molotov cocktails. One rioter was arrested.
During another Orange parade in the city of Armagh, 40 miles southwest of Belfast, police evacuated a major street called Friary Road after spotting a small bomb. It detonated before British army experts could defuse it using a remote-controlled robot. The blast caused no injuries or damage.
Scores of Catholic youths later attacked police on Friary Road with Molotov cocktails. They also hijacked and burned two cars on the road. Police arrested four rioters.
After nightfall, hijackers abandoned a car on the main street of Lurgan, a town southwest of Belfast regarded as a dissident power base. Police shut the road, but army experts weren't sure early Tuesday whether it was a car bomb or a hoax.
A similar alert forced police to seal off a bridge and divert traffic in the predominantly Catholic border village of Strabane.
No group claimed responsibility for any of the the day's violence. But police and politicians blamed IRA splinter groups that reject the underground group's 2005 disarmament.
Analysts agree that the dissidents' sporadic bombings and shootings stand no chance of forcing Northern Ireland out of the United Kingdom, the traditional IRA goal. But they do serve to embarrass and undermine Sinn Fein, the IRA-linked party that has left behind militarism in favor of seeking compromise with Protestant leaders.
"The Twelfth" officially commemorates the July 12, 1690, triumph of Protestant King William of Orange versus his Catholic rival for the English throne, James II, at the Battle of the Boyne south of Belfast. This year the parades took place on the 13th because Orangemen - who march beneath banners depicting the British crown on an open Bible - refuse to hold the holiday on a Sunday.
Orangemen once marched wherever they wanted in Northern Ireland, a state created on the back of Orange power as the predominantly Catholic rest of Ireland won independence from Britain in the early 1920s.
Catholic hostility to Protestant parades helped ignite warfare over Northern Ireland's future that claimed more than 3,600 lives from the late 1960s to mid-1990s, when paramilitary cease-fires finally took hold.
As the IRA lowered its guns, Sinn Fein activists began blocking Orangemen's traditional marching routes in several cities, towns and villages. The tactic brought Northern Ireland to the brink of civil war - and ended in broad defeat for the Orangemen, who refused to negotiate on their marching rights until it was too late.
Britain punished the Orangemen's stubbornness by imposing bans on parades that encountered the heaviest opposition from Catholics. Orangemen spent years mounting violent standoffs with British security forces in hopes of regaining lost ground, but eventually gave up.
The Crumlin Road beside Ardoyne is the only remaining parading point in Belfast that inspires recurring violence. There, the Orangemen have no obvious alternative way to march from their lodges to central Belfast and back.