The Police Service of Northern Ireland said 27 officers suffered mostly minor injuries during street clashes with more than 200 masked Irish Catholics, who threw Molotov cocktails, bricks and other makeshift weapons at rows of shield-wielding police backed by armored vehicles.
One attacker also fired at least one round from a shotgun, hitting three officers with pellets. Police said their injuries were not serious, in part because Northern Ireland riot police wear heavy protection, including helmets and head-to-toe flame retardent suits.
The Orange Order's annual July 12 marches commemorate a 17th-century battlefield victory over Irish Catholics. The mass Protestant demonstrations have always stirred sectarian passions and have fueled a four-decade conflict over Northern Ireland that has left 3,700 dead.
But marching-inspired violence has greatly reduced over the past decade amid wider peace agreements and British-imposed restrictions on where the Protestants - accompanied by so-called "kick the pope" bands of fife and drum - can march. Protestants initially opposed the restrictions with violent standoffs but resentfully accept them today.
More than 50,000 Protestants assembled Monday at 18 marching locations across this British territory of 1.8 million. They paraded under banners depicting the July 12, 1690, victory of Protestant King William of Orange versus the forces of his rival for the British throne, James II, at the Battle of the Boyne south of Belfast.
Since the early 19th century, the Orange Order has rallied Protestants under the banner of William, sword raised atop a white horse, and the symbol of a British crown atop an open Bible.
The order, which seeks to unite members of more than 50 Protestant denominations and sects, was pivotal in establishing the new state of Northern Ireland as a Protestant-dominated corner of the United Kingdom when the overwhelmingly Catholic rest of Ireland won independence from Britain in 1921.
Police said all of Monday's main processions reached their destinations peacefully, where Orangemen in business suits, orange vestments and bowler hats sat down in farm fields to gossip with friends, have lunches of small sandwiches and tea, and hear religious and political messages from their leaders.
Orange leaders declared Monday their determination to unite the two rival Protestant political parties, the Democratic Unionists and Ulster Unionists, "on the central issue that matters to the unionist people, the constitutional security of Northern Ireland."
But riot police braced for resumed trouble at night, when small groups of Orangemen return to their scattered lodges in so-called "feeder" parades and pass near hostile Catholic districts of Belfast. The worst rioting in recent years has happened at the Catholic enclave of Ardoyne in north Belfast.
Police said they arrested three suspected Irish Republican Army dissidents aged 41, 42 and 46 Monday in Ardoyne and other Catholic parts of north Belfast.
The IRA killed nearly 1,800 people in a failed 1970-1997 campaign to force Northern Ireland out of the United Kingdom. The dissidents oppose the IRA's decision to renounce violence and disarm in 2005 in support of a new Catholic-Protestant government for Northern Ireland.
Some Protestant areas also suffered violence early Monday during eve-of-parade celebrations around hundreds of makeshift bonfires.
Northern Ireland firefighters dealt with 28 incidents overnight in which bonfires threatened to engulf nearby homes, but no property was seriously damaged.
Police said seven people, including two children, were injured when a car collided with a crowd watching one bonfire near a Belfast hospital. Police said none of their injuries was critical.
"It was a scene of utter chaos afterwards," said Ruth Patterson, a Protestant city councilman and witness. "People were lying on the road screaming and there were nine ambulances there helping the injured."