TUOL SLENG, Cambodia (CNN) -- Kaing Guek Eav on Tuesday expressed sorrow for his actions 30 years ago as a prison chief for the Khmer Rouge regime, as he stood before the tribunal trying him for alleged war crimes.
Duch ran a prison where people were tortured and killed under the Khmer Rouge.
He kept his emotions in check for the most part but cracked slightly during parts of his testimony.
Earlier in the day, prosecutors continued to lay out their case against the 66-year-old former math teacher, who is better known as Duch.
Once again, prosecutors stressed how Duch actively participated in the torture and killing of some of the 15,000 prisoners at the S-21 facility.
The prison played a vital role in the widespread attack on the Cambodian population, they said.
Here, men, women and children were shackled to iron beds and tortured -- before they were beaten to death, prosecutors said.
The trial is taking place just outside the Cambodian capital, Phnom Penh.
Spectators, many of them survivors of the abuse, watched the proceedings from an auditorium separated from the courtroom by a large glass window.
Many of the victims were military officials or Communist Party members targeted for not going along with the philosophy of the ultra-Maoist Khmer Rouge movement, prosecutors said.
Duch faces charges that include crimes against humanity, war crimes, torture and murder.
The movement swept to power in 1975. Three years, eight months and 20 days later, at least 1.7 million people -- nearly one-quarter of Cambodia's population -- were dead from execution, disease, starvation and overwork, according to the Documentation Center of Cambodia.
S-21 was one of 189 similar institutions across Cambodia. Duch is the first former Khmer Rouge leader to stand trial.
The tribunal, which is made up of Cambodian and international judges, does not have the power to impose the death penalty. If convicted, Duch faces from five years to life in prison.
The trial is expected to last three or four months.
Four of the regime's former leaders, also accused of war crimes and crimes against humanity, await trial before the tribunal.
CNN's Dan Rivers contributed to this report.