Kaing Guek Eav is accused of overseeing the torture and execution of more than 15,000 men, women and children in Cambodia.
In Cambodia, final arguments are being heard in the trial of a senior Khmer Rouge leader blamed for the deaths of at least 12,000 people.
Kang Guek Eav, also known as Duch, apologized in court Wednesday. He said he will always be liable for the crimes committed under his stewardship and will always be responsible for the souls who perished at the prison he ran.
He spoke after prosecutors demanded a 40-year prison sentence for Duch's systematic killing of thousands of prisoners at the Khmer Rouge prison known as Toul Sleng, or S-21. As many as 12,000 prisoners died there - only a few ever walked out of S-21 alive.
The prosecutors did not demand a life sentence, because of the years Duch as already spent in prison and because he has shown some remorse, pleaded no contest to the charges and provided evidence against other Khmer Rouge leaders.
Prosecutors Wednesday told the court that Duch's crimes are comparable to massacres carried out under Stalin's Soviet Union and Nazi Germany, where those who did not fit in faced violence.
Co-prosecutor William Smith says under the Khmer Rouge, which ruled Cambodia from April 1975 to January 1979, prisoners were routinely beaten, faced electric shocks, had their toenails torn out, were whipped and faced water-boarding.
Surgery was performed on prisoners without anesthesia and blood was extracted from them until they lay dying.
"The closing of Kang Guek Eav, known as Duch, will also close the darkest chapter in Cambodian history," said Reach Sambath, the chief spokesman for the court. "A lot of people come to Toul Sleng and they have a lot of question marks. Now they can see the court is putting him on trial and will take him to jail."
As many as two million people perished under the ultra-Maoist government - through execution, starvation or illness. Cambodia's long running civil war and international politics meant it was not until this year that any of the Khmer Rouge leaders faced trial.
Final submission from the victims and the defense are being delivered this week before the international tribunal goes into recess. Sentencing is not expected until early next year.
Although most Khmer Rouge leaders are dead, at least four are expected to be brought to trial sometime next year.