LONDONDERRY, Northern Ireland (AP) _ Londonderry's main British Protestant group paraded atop 17th-century battlements Saturday, launching a one-sided celebration of past victories that traditionally provokes conflict with the city's Catholic majority.

But Catholic hard-liners in the nearby Bogside district agreed beforehand to stay away as about 300 Apprentice Boys in suits, bowler hats and purple vestments marched along the wall that rings central Londonderry. They were led by a red-coated flute band from Scotland that played a religious hymn, ``Abide With Me,'' as they passed near the Bogside below.

More than 15,000 Protestants from all over Northern Ireland gathered in the city for a bigger afternoon parade into Londonderry's central square and back across the Foyle River to the city's predominantly Protestant east side.

The annual gathering commemorates the 1689 survival of Londonderry's Protestant garrison during a 105-day siege by forces loyal to Catholic King James II, a seminal event in Protestant folklore. Before the marching began, a troupe of amateur actors in period costumes re-enacted the moment when English supply ships and reinforcements sailed up the Foyle to save the starving Protestants.

But a bomb threat on the main Belfast-to-Londonderry rail line delayed the arrival of several thousand Apprentice Boys and band members to Northern Ireland's second-largest city.

Police said they received several telephoned warnings late Friday claiming that a bomb had been left somewhere on the line, but didn't immediately find any devices during Saturday searches.

Two months ago, Irish Republican Army dissidents opposed to the outlawed group's 1997 cease-fire blew up a section of the main Belfast-Dublin rail line. On Thursday night, police in Londonderry tried to intercept a van carrying about 500 pounds of homemade explosive into the city, but the vehicle sped back to the nearby border with the Irish Republic and was abandoned.

Cathal Crumley, a former IRA prisoner who in June was elected as Londonderry's first mayor to come from the IRA-linked Sinn Fein party, said almost no one in the city supported the dissidents. ``They ought to disband immediately because they have no credibility and nobody wants them,'' he said.

Londonderry's long-dominant Catholic politician, Social Democratic and Labor Party leader John Hume, condemned the dissidents as ``fascists'' who were trying ``to subvert the will of the Irish people.''

Hume, a moderate who in 1996 began brokering negotiations between the Apprentice Boys and Bogsiders that produced this week's accord, said most Catholics were prepared to accept Protestant parades so long as they took place with the consent of Catholic residents.

But people remained fearful that the city's legions of rowdy youth might attack units of the Royal Ulster Constabulary, the province's predominantly Protestant police force, after the parade.

Rioting by Catholic youths following Apprentice Boys demonstrations in 1998 and 1999 caused more than $15 million in damage, mostly to the Catholic-owned businesses that now dominate central Londonderry.