The rebels opened fire on the bus Thursday afternoon while it was traveling with 29 passengers from the Rwandan capital, Kigali. The ambush took place near Kilima, 18 miles northwest of Bujumbura, the capital of this tiny, central African nation.
The vehicle overturned and was surrounded by a gang of armed rebels who started taking the valuables and clothes off the terrified passengers, said a Rwandan woman identified only as Cadeau.
"We were asked to lie down on the concrete road. When they voiced their intention to kill us, I began begging them to spare my life and my child's," Cadeau said. One of the rebels came over to her and helped her up, then ordered her to run, she said.
"Everyone was crying for mercy, but I looked their chief directly into his eyes and said 'my brother, please, let us go,' " she said.
"He said 'get up and go, and tell the military we'll kill everyone and they will not be able to do anything.' I promised to pass that on."
Another girl who was wounded in the leg had also escaped, after she told the rebels that she was Cadeau's sister.
One man, a Rwandan named Christophe, was found bleeding profusely in bushes near the site.
"When the attackers started to shoot people from the bus, I tried to escape. The rebels shot me in the back and thought I was dead," he said. "I have been hiding in the bushes since yesterday."
Army spokesman Maj. Eduard Nibigiza said 20 passengers were killed, and an Associated Press reporter counted 20 bodies in a Bujumbura morgue, including the British woman.
Among the victims was Charlotte Wilson, 27, who joined the British Voluntary Service Overseas as a science teacher at a school in neighboring Rwanda.
"VSO is in touch with Charlotte's family and our thoughts are with them at this time. Charlotte was an exceptional volunteer very bright, caring and committed to rebuilding Rwanda's education system," said Penny Lawrence, VSO's director of overseas operations, in London.
The Hutu rebels have stepped up attacks on civilians as pressure has mounted on them to sign a peace agreement with the Tutsi-led army and government on ending the seven-year civil war with a power-sharing deal.
In August, 19 Hutu and Tutsi political parties and interest groups signed the agreement that was mediated by former South African President Nelson Mandela. But the two main rebel groups have refused to sign.
Another survivor, a young man who refused to give his name, said he escaped to the bush and watched the rebels "executing the people one by one." He attempted to flee, but was shot and left for dead before he crawled for cover.
Two other vehicles driving behind the bus were also attacked, although it wasn't immeditely clear how many people were killed in those two cars.
The survivors ran to the nearest military position, about two miles away, hearing gunshots in the background as they fled.
Apart from Wilson, the rest of the passengers were Rwandans and Burundians. The bus, called Titanic, is a regular shuttle between Kigali and Bujumbura.
More than 200,000 people have been killed since 1993, when Tutsi paratroopers assassinated the first democratically-elected president, Melchior Ndadaye, a Hutu.
Since then, the Hutu rebels took up arms, demanding that the minority Tutsis give up power. The Tutsis have controlled Burundi for all but three months since the country's 1962 independence.
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