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Duch, a former S-21 prison chief, stands behind his two lawyers on Tuesday. (AFP Photo)

The former commandant of Khmer Rouge's main torture facility on Tuesday told a UN-backed tribunal that he took responsibility for the crimes committed there, and agreed to accept all the 260 crimes charged against him.

Kaing Kek Iev, known as Duch, took the stand on Tuesday and said he wanted to "apologise to the survivors of the regime and also to the families of the victims who have loved ones died so brutally at S-21."

"I would like those people to please know I would like to apologise," Duch said.

66-year old Duch commanded the group's main S-21 prison, also known as Tuol Sleng, where as many as 16-thousand men women and children are believed to have been brutalized before being sent to their deaths. Duch said he took responsibility "for crimes committed at S-21."

He was called the stand to defend himself against accusations made by the prosecution, which delivered its opening arguments on Tuesday.

The tribunal is seeking to establish responsibility for the deaths of an estimated 1.7 million from starvation, medical neglect, slave-like conditions and execution under the Khmer Rouge, whose top leader, Pol Pot, died in 1998.

He is charged with committing crimes against humanity and war crimes, as well as torture and homicide, and could face a maximum penalty of life in prison. Cambodia has no death penalty.

As well as photographs and death lists from the prison, the court was shown a film shot when the facility was found by invading Vietnamese troops in January 1979, which showed the corpses of the last victims to be killed there.

Duch, unlike the four other former Khmer Rouge leaders awaiting trial, has admitted his guilt and has asked for forgiveness.

Duch's lawyer Francois Roux objected on legal grounds to showing some footage of Tuol Sleng after it was abandoned by the Khmer Rouge, but the judges rejected his argument.

The film clearly affected one of only three living survivors of S-21. Bou Meng's talent as a painter saved him from execution, which his jailers put to use producing portraits of leader Pol Pot. But his wife who was arrested with him was killed there. In a break in proceedings Bou Meng said seeing the film reminded him of his wife, and that it "horrified" him.

He said that Duch's apology was "unacceptable."

Another S-21 survivor, Chum Mey, said when he heard Duch speak he felt "relaxed," but that he was "very frustrated with his lawyer."

Duch has been in detention since he was discovered in 1999 by British journalist Nic Dunlop in the Cambodian countryside, where he had been living under an assumed name.

Dunlop, who attended Tuesday's hearing, said it was "interesting" to see Duch's reaction to evidence being played out in the trial. "As far as I can see there's been absolutely no reaction from him and that's probably been the most interesting aspect for me," Dunlop said.

Information for this report was provided by APTN.