Prosecutors in the genocide trial of a former Khmer Rouge prison chief have demanded a 40-year jail sentence for a man they said took innocent lives and spread terror across Cambodia.

Kaing Guek Eav commanded the notorious S-21 prison, where those accused of disloyalty to the xenophobic communist regime were held.

He oversaw the torture and execution of about 16,000 men, women and children during the Khmer Rouge's 1975-79 rule.

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.

Co-prosecutor William Smith told the court that Kaing Guek Eav, also known as Duch (pronounced Doik), acknowledged his guilt and had given evidence against other Khmer Rouge leaders. But he said Duch must be held accountable for his "unrelenting brutality" at S-21.

"The sentence to be imposed by this trial chamber should be 40 years' imprisonment," Mr Smith said, adding that five years had been taken off the request for his co-operation and five more for time already served.

Duch is expected to be sentenced early next year. He showed no reaction when the sentence recommendation was read. If the tribunal finds him guilty, the former teacher could still face a maximum penalty of life in prison. Cambodia has no death penalty.

Duch is charged with crimes against humanity, war crimes, murder and torture.

He has denied personally killing or torturing the S-21 prisoners, and said he acted with reluctance on orders from his superiors, saying he feared for the safely of his family and himself. Survivors of the S-21 described being routinely beaten, receiving electric shocks, having their toenails torn out and being waterboarded. Surgery was performed on detainees without anaesthetic.

In his testimony, Duch has accepted responsibility for his role in overseeing the prison and asked for forgiveness from victims' families. He also told the court he was ready to accept heavy punishment for his actions.