Security forces Wednesday surrounded a bus filled with nuns and schoolgirls that was hijacked by armed guerrillas as it headed for a Mass to be celebrated by Pope John Paul II.

Helicopters and armed cars blanketed central Maseru as 100 armed soldiers and police set up roadblocks and kept spectators at least four blocks from the bus, parked on the grass outside the British High Commission, or embassy.Police denied an earlier report that some of the approximately 70 captives, including eight nuns and 36 children, 16 women and 10 men, had been released.

Bad weather and mechanical trouble, meanwhile, forced the plane carrying John Paul from Botswana to divert from Maseru to Johannesburg in South Africa, where the pontiff was informed of the hijacking but made no comment. The pope later left by road for Maseru, where he will celebrate a Mass on Thursday.

Official Lesotho Radio said a group of "fanatics" had taken over the bus of papal pilgrims Tuesday. Diplomats said the guerrillas were believed to be members of the dissident Lesotho Liberation Army.

Four hijackers posing as pilgrims boarded the bus Tuesday at the start of its journey to Maseru from rural Lesotho, the radio said. The rebels were armed with an automatic rifle, a pistol and two grenades.

Lesotho Radio said the hijackers' request to meet British High Commissioner John Edwards had been refused.

The hijackers asked Wednesday that a Catholic priest be taken to the bus. Unofficial sources said they believed the priest would open negotiations between the hijackers and Lesotho's military rulers.

At Johannesburg airport, Foreign Minister R.F. Botha said South African forces had entered Lesotho to help free the hostages.

Earlier, three police armored personnel carriers and some soldiers were seen on the South African side of the border post just outside Maseru.

South Africa had been omitted from the pope's five-nation southern Africa tour after South African bishops said it was not an appropriate time for a visit. The pope has spoken out against apartheid, the system of racial segregation in South Africa.

Witnesses said guerrillas took over the bus at Mazenod, outside Maseru. They said police surrounded the bus but later moved back and let it proceed to Maseru.