(CNN) -- Closing argument wrapped up Friday in the trial of a former Khmer Rouge prison chief accused of killing thousands of people three decades ago.
Charges against Kaing Guek Eav, known as Duch, include war crimes, crimes against humanity and torture during the communist regime's rule from 1975 to 1979.
He sought an acquittal during the hearing, said U.N. spokeswoman Yuko Maeda.
Prosecutors in the U.N.-backed Cambodian tribunal say Duch 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, is 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-67-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.
Duch's months-long trial has been held outside the Cambodian capital, Phnom Penh. Spectators, many of them survivors of the abuse, have watched the proceedings from an auditorium separated from the courtroom by a large glass window to prevent revenge attacks.
Duch has repeatedly apologized for the atrocities and 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 during opening statements in March.
"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, spokesman for the Extraordinary Chambers in the Courts of Cambodia.
A verdict is expected early next year, Olsen said. The tribunal is made up of Cambodian and international judges.
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. Prosecutors have called for a 40-year sentence.
The former Khmer Rouge leader said he was just an instrument, with no choice but to follow orders from a regime that was determined to destroy all its enemies, Olsen said.
Duch asked the court to consider his actions in context, saying the torture and killings were inevitable, Olsen said.
Many of the victims were military officials or members of the Communist Party who were targeted for not going along with the philosophy of the ultra-Maoist Khmer Rouge movement.
S-21 was one of 189 similar institutions across Cambodia. And Duch is the first former Khmer Rouge leader to stand trial.
Four other former leaders await trial before the tribunal, also accused of war crimes and crimes against humanity.
CNN's Saeed Ahmed contributed to this report.