LISBURN, Northern Ireland (AP) _ The government admitted Thursday that a security lapse allowed Irish guerrillas to kill six British soldiers by planting a bomb on their van while they were running in a charity race.

The Irish Republican Army claimed responsibility and vowed ''to wage unceasing war'' against British forces in Northern Ireland.

Townspeople in Lisburn, seven miles outside Belfast, gathered behind police barriers in small, silent groups to gaze at the burned-out hulk of the bombed van.

Others later approached the site and left bouquets.

''We're just expressing sympathy with the soldiers that are in this country trying to protect us and this is what they're getting,'' one unidentified young woman told British Broadcasting Corp. TV as she put down a bunch of flowers.

''We just hope that their families understand that it's not everyone here that's like that,'' she said.

Sean McManus, chairman of the IRA's legal political wing Sinn Fein, appealed to families of British soldiers serving in the province to press their government to ''withdraw from Ireland and bring an end to the tragic deaths resulting from Britain's partition of our country.''

''We regret all deaths and earnestly desire peace, but the deaths of the six British soldiers on Wednesday night are part of the price of the British government's intransigence,'' McManus said in statement issued Thursday in Dublin, capital of the Irish Republic.

A half-marathon for charity had just ended Wednesday evening at the close of a warm, sunny day in this garrison town, headquarters for the British army in Northern Ireland. The exploding bomb turned the van into a fireball and scattered torn, burning bodies onto the street.

''I saw two bodies lying in the middle of the road,'' said Nigel Sands, 19. ''One of them had no legs, and one of them was burning. It was badly mutilated. It seems they were blown from the van. There was panic and people were running everywhere.''

Ten civilians were wounded, including a 2-year-old boy and a man of 80, but authorities said none was hurt seriously.

Political leaders in Britain and Ireland joined prominent Protestants and Roman Catholics of Northern Ireland on Thursday in denouncing the attack.

Prime Minister Margaret Thatcher said in the House of Commons the bombing was a ''terrible atrocity'' and reiterated the government's determination ''to defeat the terrorism of the IRA, which shoot and bomb to kill.''

Northern Ireland Secretary Tom King, the government's top official in the province, conferred with army commanders in Lisburn before flying to London to report to Parliament.

''There is a vital need for everybody to pay the closest attention to personal security,'' he told reporters. ''It's absoluely standard practice. I think, tragically, it does seem clear that this was not followed in this case.''

The soldiers had traveled to Lisburn from their barracks in Londonderry, in the north of the province, in a blue van not marked as an army vehicle. The bomb was believed to contain Czech-made explosives, and experts were trying to determine how it was detonated.

An editorial in the independent Belfast Telegraph, the province's largest newspaper, said the ''professionalism in fixing the bomb without detection, in an open car park, is chilling.''

It noted that soldiers were barred from participating in civilian events after two were killed when they blundered into an IRA funeral cortege in March. The soldiers had been permitted to join the Lisburn event, however, and the paper asked how the IRA found out.

''Somehow the word must have leaked out,'' it said. ''The bomb was ready and the van was identified. The level of intelligence needed was considerable, in a garrison town.''

An IRA statement delivered to newspapers and other media in Belfast said it planted a seven-pound bomb for what it described as an ''execution.''

It was the highest death toll since an IRA bomb in Enniskillen last November killed 11 Protestant civilians, and the highest army toll since 11 soldiers and six civilians were killed in a 1982 pub bombing near Londonderry.

''Until that (British) presence is removed and the Irish people as a unit can exercise our right to national self-determination, the IRA will continue to wage unceasing war against the British crown forces and the British colonial apparatus,'' the IRA statement said.

Britain says it will stay as long as the majority wishes. Most Protestants, who outnumber Catholics 3-2, are loyal to Britain and oppose union with the overwhelmingly Catholic republic.

The army identified the dead soldiers as: Cpl. Ian Metcalfe, 36; Lance Cpl. Graham Patrick Lambie, who would have been 23 on June 29; Lance Cpl. William John Paterson, 22; Sgt. Michael James Winkler, 31; Signaller Mark Robert Clavey, 24, and Lance Cpl. Derek Walter Green, 20.