PHNOM PENH, Feb 17 (Reuters) - Survivors of Cambodia's Khmer Rouge atrocities reacted with pain, anger and relief on Tuesday as they watched Pol Pot's chief torturer in the dock, 30 years after the fall of a regime blamed for 1.7 million deaths.

Nearly 800 people, including saffron-robed Buddhist monks who were persecuted during the 1975-79 Khmer Rouge era, flocked to see the first formal trial of a senior Pol Pot cadre by a U.N.-backed tribunal.

"When I see Duch, my anger comes back," Phok Khan, 56, told Reuters during a break in the trial being held on the outskirts of Phnom Penh.

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Duch, also known as Kaing Guek Eav, has admitted to atrocities at S-21, the former school where up to 14,000 "enemies of the revolution" were tortured before being beaten to death in the Killing Fields outside the capital.

Vann Nath was among a handful who survived.

The 63-year-old, white-haired artist, who was beaten by S-21 guards but escaped death due to his talent for painting portraits of Pol Pot, said he had mixed emotions after seeing his former tormentor on trial.

"This is the day we have waited for for 30 years. But I don't know if it will end my suffering," he told reporters during a break in the nationally televised hearing.

Duch sat impassively in the wood-panelled court wearing a blue, long-sleeved shirt and sipping water.

He occasionally donned reading glasses, an ironic twist since the Khmer Rouge targeted those who wore spectacles, seeing that as a mark of the intelligentsia and enemies of the revolution.

Duch, now a born-again Christian, has confessed to his crimes but says he was just following orders. The French-speaking teacher-turned-torturer did not address the court on Tuesday. A court guard sat nearby as the frail ex-commandant wore headphones and took notes while the five-judge panel considered procedural motions.

Outside the glass-enclosed trial chamber, hundreds of Cambodians sat in the public gallery, some listening intently on headphones. A few dozed as Cambodian and international lawyers argued over procedural issues.

HEINOUS ACTS

Them Khean said he had travelled 240 kms (150 miles) from the province of Kompong Thom where Duch and Pol Pot were born.

"I want to see what Duch has to say about his past crimes," said Khean, 65, who lost 10 relatives to starvation and torture.

Va Boeurn, 66, said he wanted to see Duch's face to try to understand how a human being could commit such heinous acts.

"What does his face look like? Why was he so brutal?" asked Boeurn, who lost seven family members during Pol Pol's reign of terror, aimed at building an agrarian utopia.

Most Cambodians lost relatives during four years of killings, starvation and overwork before the Vietnamese army ousted Pol Pot in 1979. Pol Pot died in 1998 in the last Khmer Rouge stronghold of Anlong Veng on the Thai border.

"Duch is in front of the victims. That is all that matters today. It is a moment of importance," said Pierre Olivier-Sure, a lawyer for 15 victims. (Editing by Darren Schuettler)