The Khmer Rouge prison boss who admitted responsibility for the torture and murder of more than 12,000 people today stunned a war crimes court by asking to be acquitted and released.

On the last day of a nine-month trial, Kaing Guek Eav, known as Comrade Duch, asked the judges to consider his co-operation with the court and the 10 years he had already served in jail and set him free.

In the last sentence of his final summing up, he said: "I would ask the chamber to release me, thank you very much."

The extraordinary request came just two days after he told the court he was ultimately accountable for the deaths that occurred while he headed the Khmer Rouge's Tuol Sleng prison in Phnom Penh. "I am solely and individually responsible for the loss of at least 12,380 lives," he said.

This morning an astounded bench asked Duch's lawyer, Kar Savuth, to clarify his statement, asking if his plea to be released was a request for acquittal.

"Release means acquittal," the court was told.

Duch's request enraged Bou Meng, one of only a dozen prisoners to walk out of Tuol Sleng alive. He stormed from the public gallery, describing Duch's plea as an insult to his wife's memory.

"I could not accept the request for the release by Duch, because many people, including my wife, have been killed during the Khmer Rouge time. He cannot step on the victims like this."

The prosecutor, William Smith, said outside court that he was surprised by Duch's last-minute change of heart. "The fact that he entered a request for an acquittal reinforces in our mind that his remorse is limited."

The prosecution has asked for 40 year's jail for Duch, 67. He will be sentenced next year.

Between 1975 and 1979 Tuol Sleng was the centrepiece of the Khmer Rouge's brutal security regime. "Enemies of the party" were tortured – shocked, whipped, beaten, and mutilated – into false confessions, then bludgeoned to death and buried in mass graves.

Outside court, Dara Chey, a student who lost four relatives during the Khmer Rouge years, said Duch's request for acquittal cast doubt on his earlier apology. "I do not believe him when he says he is sorry any more. He is just trying to get out of jail. He should never be allowed out. Cambodians will not be happy if he ever walks free."

This week Duch asked to be allowed to apologise in person to his victims' families. No family members of victims, or victims' groups, have said they want to meet with Duch.

The Khmer Rouge leader, Pol Pot, died in 1998. The joint trial of four more senior Khmer Rouge leaders is expected to start in mid-2011, while the court is considering whether to open cases against five other former Khmer Rouge cadres.