PHNOM PENH - A senior Khmer Rouge prison guard on Thursday told a war crimes tribunal he was forced to send thousands of detainees to an execution site, where they were brutally killed and their bodies thrown into mass graves.

Him Huy, 54, a guard at Phnom Penh's notorious S-21 prison, said he was ordered by Pol Pot's chief jailor to transport prisoners to a rice field where they were stripped naked and beaten with clubs as they bled to death.

All prisoners were blindfolded so they did not know where they were taken and their hands were tied up to prevent them from contesting us, Huy told the joint United Nations-Cambodian tribunal.

They were asked to sit on the edge of the pits and they were struck with stick on their necks, he said, his voice breaking as he gave his harrowing account of the Choeung Ek executions.

Their throats were slashed before we removed their handcuffs and clothes, and they were thrown into the pits.

Huy was testifying against S-21 chief Duch, whose real name is Kaing Guek Eav, the first of the five indicted former Khmer Rouge cadres to face trial.

WAR CRIMES

Duch faces a maximum sentence of life in prison if convicted on charges of war crimes, crimes against humanity, torture and murder.

Huy said he saw the charred bodies of four Westerners on a pile of burning tires a few block away from S-21, where he said about 100 children were detained inside a compound with their mothers.

All were later executed.

No one could dare to do anything without Duch's approval, Huy added, as Duch listened attentively.

Choeung Ek, 17km south of Phnom Penh, is now a memorial to the horrors of the Khmer Rouge's 1975-1979 killing fields reign of terror, when 1.7 million Cambodians were killed.

The burial site, one of 343 across the country, is covered with 129 graves, with the skulls of about 5,000 Cambodians on display in a stupa.

Duch, who has been detained since 1999, wept repeatedly as he prayed before the skulls of his victims during a visit to Choeung Ek in February last year.

The tribunal has also indicted second-in-command Nuon Chea, former President Khieu Samphan and ex-foreign minister Ieng Sary and his wife, both lifelong friends of Pol Pot.

Duch has admitted involvement in the killings of 14,000 people at the S-21 prison, but says he was only following orders.

The others facing trial have denied knowledge of the atrocities, while Brother Number One Pol Pot, the architect of the ultra-Maoist revolution, died in 1998 near the Thai-Cambodia border.
(Reporting by Ek Madra; Editing by Martin Petty)