Former Khmer Rouge prison commander Kaing Guek Eav, also known as Duch, sits in the courtroom at the UN-backed tribunal on Monday in Phnom Penh, Cambodia. (Mak Remissa/Pool/Associated Press)

The man charged in the deaths of as many as 16,000 at the Khmer Rouge's most notorious torture facility expressed sorrow for his actions before a genocide tribunal on Tuesday and said he'd take responsibility for the crimes committed at the prison.

Kaing Guek Eav, commonly known as Duch, told the UN-assisted tribunal on Tuesday that he wanted to convey "regretfulness and heartfelt sorrow for all crimes" committed during the Khmer Rouge's rule in Cambodia in the 1970s.

Duch, 66, served as commander of the regime's main prison in Phnom Penh, known as S-21, or Tuol Sleng. His job was to obtain confessions of counter-revolutionary activity before sending the prisoners — men, women and children — to their deaths.

Duch is charged with committing crimes against humanity and war crimes, as well as torture and homicide for his role running the facility where as many as 16,000 people were brutally tortured before being executed.

"I recognize that I am responsible for the crimes committed," Duch testified, reading from a prepared statement.

"May I be permitted to apologize to the survivors of the regime and families of the victims who had loved ones who died brutally at S-21. I would like you to forgive me," Duch told the panel of five judges and citizens who gathered in the public gallery of the specially constructed courthouse on the outskirts of Phnom Penh.

'I am just a scapegoat'

Duch said he took responsibility for the crimes committed at S-21 but that he was a scapegoat for the atrocities committed by the Khmer Rouge.

"I am just a scapegoat — a person who played a role in the killings," he said.

Duch is the first of five officials from Pol Pot's hardcore communist regime that from 1975 to 1979 turned Cambodia into a vast labour camp and killing field that left an estimated 1.7 million people dead. If found guilty, Duch could face a maximum sentence of life in prison. There is no death penalty in Cambodia.

Duch is expected to be a key witness in the future trials of Khieu Samphan, the group's former head of state; Ieng Sary, its foreign minister; his wife, Ieng Thirith, who was minister for social affairs; and Nuon Chea, the government's chief ideologue.

The four other defendants have denied committing any crimes or having any knowledge of the atrocities of the Khmer Rouge.

Defendants at the tribunal do not enter pleas. Tribunal officials said the primary goal of the trial is to determine the facts of what happened during Khmer Rouge rule.

"I cannot accept his apology because too many people were killed," Phoung Savy, 49, who lost five relatives, told Reuters.

Buoy Phalkun, 21, who watched the trial from the public gallery, said Duch appeared sincere.

"Maybe we should be lenient because he admitted to the crimes," he said.

History demands justice: prosecutor

Co-prosecutor Chea Leang vowed to get justice for the victims of the regime.

"For 30 years, the survivors of Democratic Kampuchea have been waiting for accountability. For 30 years, a generation of Cambodians have been struggling to get answers for their fate," Leang said.

"Justice will be done," she said. "History demands it."

Duch's began Monday with the reading of 45-page indictment that described in gruesome detail how prisoners were tortured and killed.

Duch, a former teacher, disappeared after the group fell from power in 1979. He lived under two different names, returned to teaching and converted to Christianity.

He was discovered by a British journalist in the Cambodian countryside in 1999 and has been held in detention awaiting trial since.

With files from the Associated Press