PHNOM PENH - The prison chief of Cambodia's brutal Khmer Rouge begged victims for forgiveness Tuesday after dramatically admitting responsibility for crimes under the regime that killed two million people.

Duch told a UN-backed court that he was a scapegoat for senior members of the ultra-communist movement and insisted that he followed orders at the notorious jail he ran to protect his own family.

The 66-year-old faces trial on charges of crimes against humanity and war crimes over the extermination of around 15,000 people between 1975 and 1979 at the feared Tuol Sleng jail, also known as S-21.

"I would like to emphasize that I am responsible for the crimes committed at S21, especially the torture and execution of people there," said Duch, who is also known as Kaing Guek Eav.

"May I be permitted to apologise to the survivors of the regime and also the loved ones of those who died brutally during the regime," he said, adding that he felt "regret and heartfelt sorrow."

Duch, who became a born-again Christian before his arrest in 1999, added: "My current plea is that I would like you to please leave me an open window to seek forgiveness," he added.

But the former maths teacher described himself as a "scapegoat", saying had not held a senior role in the brutal 1975-1979 regime led by Pol Pot.

"In those times, I regarded the life of my family as more important than those detained at S21. Although I knew my orders were criminal, I never dared to challenge top authorities," he said.

Duch, who faces a possible life sentence, is the first and arguably the most junior of five Khmer Rouge leaders who are in the custody of the court. The other four are due to go on trial later this year.

Prosecutors earlier described him as playing a central role in the regime's enforcement of its iron-fisted rule as it tried to force Cambodia back to an agrarian "Year Zero" with disastrous results.

"S21 formed an integral and indeed vital role in a widespread attack on the population of Cambodia. The accused's crimes were part of this attack," co-prosecutor Chea Leang told the court, as Duch jotted notes in the dock.

Inmates had toenails and fingernails pulled out, had plastic bags tied over their heads, were stripped naked and had electric shocks administered to their genitals, international co-prosecutor Robert Petit said.

"From the day it claimed its first victim, the policy was that no one could leave S21 alive," Petit told the court.

Most prisoners were killed by a blow to the base of the neck with a steel club, then had their bellies sliced open, Petit said.

But he added a former Tuol Sleng guard will testify many prisoners were drained of their blood. "Victims would be strapped to a bed, hooked up to an IV and literally have their life drained out of them," Petit said.

During a brief recess, Tuol Sleng survivor Bou Meng told reporters that video footage shown by prosecutors, which included a corpse chained to a bed, reminded him of how he lost his wife at the prison.

"I cannot forgive Duch because of my wife's life. I want to beat him to death, but I respect the law and now is the time to use it," said Bou Meng, who was spared because his skills as an artist were deemed useful to the regime.

Pol Pot died under house arrest in 1998, and many in Cambodia believe the UN-sponsored tribunal is the last chance to bring those regime figures still alive to justice.

The tribunal, formed in 2006 after nearly a decade of wrangling between the United Nations and Cambodian government, has faced controversy over allegations of corruption and political interference.

Cambodian premier Hun Sen warned on Tuesday that his country would be plunged back into civil war if the court pursued more former Khmer Rouge leaders as international prosecutors wish.

"I would like to say that I prefer for this court to fail... I won't allow war to re-occur in Cambodia," said Hun Sen, himself a former Khmer Rouge commander who defected to join Vietnamese troops who ousted the regime in 1979.