By Ek Madra

CHOEUNG EK, Cambodia, Feb 26 (Reuters) - The chief torturer under the Khmer Rouge "Killing Fields" regime wept and prayed on Tuesday as he led the judges who will try him for crimes against humanity around the mass graves for some of its victims.

Duch, also known as Kaing Guek Eav, accompanied 80 judges, lawyers and other officials of a U.N.-backed tribunal to the 129 graves, uncovered after a Vietnamese invasion sent the Khmer Rouge back to the jungles in 1979.

"I saw Duch kneel in front of the trees where Khmer Rouge soldiers smashed children to death," a policeman told reporters after the four-hour tour.

"He cried and apologised to the victims" in the former ricefields outside Phnom Penh, he said.

Stacks of excavated skulls mark the area.

Some of the victims were from the regime's S-21 prison at the former Tuol Sleng high school in Phnom Penh run by Duch, now 66.

About 14,000 people -- including a few foreigners accused of being CIA spies -- went into the jail to be tortured into confessing to working against a regime deemed responsible for the deaths of 1.7 million people.

Only a handful emerged alive.

"Duch expressed his sadness and shed tears two to three times," tribunal spokesman Reach Sambath said. "He held his palms together to pay respect to the victims in front of the shrine of skulls."

Duch, the first senior Khmer Rouge official to be detained, was to lead court officials on a tour of Tuol Sleng on Wednesday.

"This is just one more piece in building a case file. It can be very useful in court to have a visual representation of the site in question," Australian court official Helen Jarvis said.

Tuol Sleng is now a shrine to those killed by the Khmer Rouge, who also eradicated potential opponents of their back to "Year Zero" revolution to produce an agrarian utopia through overwork, starvation and disease.

Detained in 1999 and now a Christian, Duch is expected to be a key witness in the trials of "Brother Number Two" Nuon Chea, Khmer Rouge leader Pol Pot's right hand man, Khieu Samphan, president under the regime, Ieng Sary, its foreign minister, and his wife.

"He could not have committed those crimes alone," Duch lawyer Kar Savuth said. "He took orders from the top leaders."

Many Cambodians want to hear what Duch will have to say in trials expected to start in July. The defendants face a maximum of life in prison.

"I still do not understand why Duch jailed me, killed my wife and our baby," said Chum Manh, 78, one of the few survivors of Tuol Sleng.

Nuon Chea is accused of playing a central role in atrocities by the Khmer Rouge during their 1975-1979 rule, which they began by driving everyone out of the cities with whatever they could carry.

He was arrested last year along with Ieng Sary and his wife, lifelong friends of Pol Pot.

Pol Pot died in 1998 in the final Khmer Rouge redoubt of Anlong Veng. (Editing by Michael Battye and Sonya Hepinstall)

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