(CNN) -- A U.N.-backed Cambodian court started closing arguments Monday in the trial of a former Khmer Rouge prison chief accused of killing thousands of people three decades ago.
Kaing Guek Eav, known as Duch, faces charges that include war crimes, crimes against humanity and torture during the communist regime's rule from 1975 to 1979.
Prosecutors say he oversaw the torture and killing of more than 15,000 people and actively took part in some cases.
The Khmer Rouge, led by Pol Pot, was blamed for one of the worst horrors of the 20th century. At least 1.7 million people -- nearly one-quarter of Cambodia's population -- died under the regime, from execution, disease, starvation and overwork.
The now-66-year-old Duch ran S-21, a prison converted from a school. At the prison, men, women and children were shackled to iron beds and tortured before they were beaten to death, prosecutors said.
About 90 survivors attended the proceedings Monday outside the Cambodian capital, Phnom Penh. In the courtroom, Duch was separated by a large bulletproof glass window to prevent revenge attacks.
Duch has repeatedly apologized for the atrocities and, during opening statements in March, pledged to work with court officials to resolve the case.
Cooperation is the best "remedy that can help me to relieve all of the sorrow of the crimes that I have committed," he said at the time.
"I will answer all questions you may ask me in the court ... and also the questions posed by the civil parties, and based on the evidence."
The former math teacher, now a born-again Christian, has admitted to about two-thirds of the charges, said Lars Olsen, a U.N. spokesman.
Closing arguments end later this week and a verdict is expected early next year, Olsen said.
Duch faces five years to life in prison if convicted by the tribunal, which does not have the power to impose the death penalty, Olsen said.
CNN's Faith Karimi contributed to this report.