Kaing Guek Eav, or Comrade Duch, has acknowledged guilt
Former Khmer Rouge prison chief Duch should be jailed for 40 years, a prosecutor has told Cambodia's UN-backed war crimes court.
Duch, whose real name is Kaing Guek Eav, had overseen the deaths of 15,000 people at Tuol Sleng jail in the 1970s, the court heard.
In a closing statement, Duch apologised to his victims but said he had not carried out the massacres alone.
The tribunal is not expected to give a verdict before early next year.
Up to two million Cambodians died under Pol Pot's brutal Khmer Rouge rule in the 1970s.
Co-prosecutor Bill Smith told the court on the outskirts of Phnom Penh that 67-year-old Duch had acknowledged his guilt and had given evidence against other Khmer Rouge leaders but must be held accountable for his "unrelenting brutality" at the prison.
A sentence of 45 years should be imposed, but five years had been deducted to take account of Duch's co-operation and time he had already served, Mr Smith said.
The BBC's Guy Delauney, in Cambodia, said the prosecution had pulled no punches, describing the "unimaginable brutality" of conditions in Tuol Sleng prison.
"Your honours should be mindful of the dreams and opportunities that were denied," Mr Smith said of those killed in the prison, and "the loss and suffering of the families of the victims who are still suffering to this very day."
As the prosecution finished its arguments and gave its recommendations, Duch showed no reaction.
He then began his concluding statement, offering an apology for his crimes.
"In order to express my most excruciating remorse, I have fully and sincerely co-operated with the court whenever it is needed of me," he said.
"I acknowledge that I was a member of the Pol Pot force and accordingly I am... psychologically accountable to the entire Cambodian population for the souls of those who perished."
But he said he had been forced to commit the atrocities he did. "I could do nothing to help," he said. "Pol Pot regarded these people as thorns in his eyes."
The notorious S-21 jail is now a genocide museum
As many as 17,000 inmates are thought to have passed through the gates of the prison, also known as S-21.
The vast majority were tortured, forced to "confess" to crimes against the regime and then put to death at the so-called killing fields just outside Phnom Penh. The prison is now a genocide museum.
Hundreds of Cambodians have attended the specially built courtroom and Duch's tribunal is being broadcast live on all the country's main TV stations.
Duch is the first of five leading Khmer Rouge figures to face the UN-backed tribunal.
The joint trial of four other - more senior - Khmer Rouge leaders is expected to start in 2011.
The court is also investigating whether to open more cases against five other former Khmer Rouge officials.