TUOL SLENG, Cambodia (CNN) -- Kaing Guek Eav is an elderly former math teacher and a born-again Christian.
It's thought 15,000 men, women and children died after being shackled to iron beds and tortured in Tuol Sleng.
He is also -- prosecutors contend -- a former prison chief with Cambodia's Khmer Rouge movement who oversaw the torture and killing of more than 15,000 men, women and children three decades ago.
The trial of the 66-year-old man, better known as Duch, began Monday in front of a U.N.-backed tribunal just outside the Cambodian capital, Phnom Penh.
Duch faces charges that include crimes against humanity, war crimes, torture and murder. Watch Duch in the dock »
Spectators, many of them survivors of the abuse, watched the start of the trial from an auditorium separated from the courtroom by a large glass window.
While he has admitted his role in the Khmer Rouge's genocidal reign, Duch won't be spared weeks of evidence, much expected to be shocking. Watch more on the evidence to be presented »
The prison he headed was a converted school, Tuol Sleng, that the regime renamed S-21.
Here, men, women and children were shackled to iron beds and tortured -- before they were electrocuted, beaten and whipped to death, prosecutors said.
Prisoners were meticulously photographed before they were killed. See inside the prison and hear of its bloody past »
"It all seems so fresh," said Norng Champhal, who was a starving little boy when invading Vietnamese forces entered the prison. He had been separated from his mother after one night in the prison and never saw her again.
"It's hard to control my feelings when I see this," he said, as he watched footage of the prison taken 30 years ago. "I wonder whether my parents were tortured like these people," he said.
The Khmer Rouge swept to power in 1975. Three years, eight months and 20 days later, at least 1.7 million people -- nearly one-quarter of Cambodia's population -- were dead from execution, disease, starvation and overwork, according to the Documentation Center of Cambodia.
The organization has been at the forefront of recording the atrocities committed during the Khmer Rouge regime. Pol Pot and his followers envisioned a return to a nation of noble peasant farmers and sought to purge city dwellers, the rich, the religious, the educated, anyone from a different ethnic group.
Tuol Sleng was one of 189 similar institutions across Cambodia. Duch is the first former Khmer Rouge leader to stand trial.
The tribunal, which is made up of Cambodian and international judges, does not have the power to impose the death penalty. If convicted, Duch faces from five years to life in prison.
The trial is expected to last three or four months.
"Probably the most important thing about this court is: even after 35 years, you are still not going to get away with it. That is the message," said Chief Prosecutor Robert Petit.
When proceedings began last month, more than 500 people -- including three survivors from the prison Duch ran -- filled the tribunal. About 50 people came from Kampong Thom province, the birthplace of now-dead Khmer Rouge leader Pol Pot.
"I couldn't sleep last night. I was dreaming about my time at S-21," Vann Nath, one of the few survivors of the prison run by Duch, told The Phnom Penh Post last month.
Even though Duch was not a senior leader with the movement, many Cambodians were relieved that one of the regime's former leaders was facing justice, said Youk Chhang, head of the Documentation Center of Cambodia.
"I think there is a feeling of, well you know, finally -- now it's finally happening after all these years of waiting -- hearing, fighting, negotiating," he told CNN last month. "People have that kind of sense of relief that it's now moving. When I ask people around the center today, people say, 'Oh, it's about time.'"
Four of the regime's former leaders, also accused of war crimes and crimes against humanity, await trial before the tribunal. Pol Pot died in 1998.
CNN's Dan Rivers and Miranda Leitsinger contributed to this report.