BELFAST -- Tens of thousands of Protestant Orangemen marched through tense and rain-drenched Northern Ireland towns Tuesday to celebrate a centuries-old victory over Roman Catholics.


The marchers from Northern Ireland's main Protestant secret society, the Orange Order, brought the British-ruled province to a standstill to commemorate "The Twelfth," the day when 304 years ago a Protestant army under King William III routed the Catholic forces of King James II.


The commemorations form the most important day in the pro-British "loyalist" calendar, when hardline Protestant resistance to change is demonstrated under the guard of armed soldiers and police.


"We reject Catholic domination. The (Irish) Republic is ruled by Rome and we will not be ruled by Rome," said David Moore, a marcher from Belfast, who wore a black suit, Orange Order tie and collarette.


Most Protestants favor Northern Ireland's continued union with Britain, opinion polls show, while most Catholics want the province reunified with the independent Irish Republic. The outlawed Irish Republican Army, waging a 24-year campaign against British rule, draws its support from the province's Catholic minority.


Elsewhere Tuesday, an IRA mortar and gun attack on a British army base in Newtownhamilton, 70 kilometers southwest of Belfast, forced a departing army helicopter to crash-land but no one aboard was believed seriously hurt, police said.


Soldiers in Belfast and Londonderry, Northern Ireland's second-largest and mostly Catholic city 100 kilometers west of Belfast, erected huge screens early Tuesday to prevent Catholics from seeing the Orange parades as they passed by their neighborhoods.


Security forces braced for fresh street violence after Catholic youths overnight hurled gasoline bombs and bricks over one of the iron fences separating Catholic and Protestant neighborhoods in north Belfast.


Protestants standing around one of scores of celebratory "Twelfth" bonfires responded with their own hand-thrown missiles, and police broke up the riot with volleys of plastic bullets.


No one was reported injured, police said, though people wounded in Northern Ireland rioting often seek private medical care to avoid the possibility of arrest. Leaders of the Orange Order said they hoped there would be no retribution for Monday's IRA killing of leading loyalist Ray Smallwoods.


Smallwoods, 44, the chairman of a fringe political party allied with the outlawed Ulster Defense Association, was shot dead outside his home in Lisburn, 19 kilometers southwest of Belfast.


He was jailed in the 1980s for trying to kill Catholic civil-rights activist Bernadette Devlin McAliskey.