The former prison chief whose trial by Cambodia's genocide tribunal is coming to an end was a willing, even enthusiastic, participant in atrocities committed by the Khmer Rouge, prosecutors charged Tuesday.

The prosecution, making its closing arguments, is demanding a lengthy prison term for Kaing Guek Eav, who commanded S-21 prison when the communist group held power in 1975-79, and oversaw the torture and execution there of about 16,000 men, women and children accused of disloyalty.

Some 1.7 million Cambodians died of torture, execution, disease and starvation during the Khmer Rouge's rule, during which the Maoist ideologues emptied cities and forced virtually the entire population to work on farm collectives.

Kaing Guek Eav, also known as Duch (pronounced DOIK), is charged with crimes against humanity, war crimes, murder and torture. He has denied personally killing or torturing the S-21 prisoners, and testified that he acted with reluctance on orders from his superiors, saying he feared for the safely of his family and himself.

"Your honors, when you review the evidence of the accused's efficiency, initiative, dedication, drive, enthusiasm and zeal, there is absolutely no doubt that, as misguided as he was, he was a fully willing participant in these crimes," Australian co-prosecutor William Smith told the U.N.-assisted tribunal.

Smith said no mercy was shown when Duch's friends and former colleagues were brought into the prison. He taught and directed his staff to achieve economies of scale by capturing and killing as many of the regime's enemies as possible.

Survivors of the S-21 have described how they were routinely beaten, received electric shocks, had their toenails torn out, and were waterboarded. Surgeries were performed on detainees without anesthetics.

Duch's believed in communist and revolutionary ideals and the Khmer Rouge trusted him to carry out the grisliest of orders, said Smith.

"It was clear the accused was proud to receive instructions and was proud to pass them on," he said. "He believed in the party and its ideals."

Dressed in a neatly ironed white shirt and beige trousers, Duch sat passively in the dock, occasionally raising his eyes toward the ceiling and sometimes scratching his chin as prosecutors accused him trying to mislead the court.

If the tribunal finds him guilty, the former schoolteacher faces a maximum penalty of life in prison. Cambodia has no death penalty. Prosecutors did not specify Tuesday how much time he should serve.