A former Cambodian prison chief known as Duch takes the stand at the U.N.-backed war crimes tribunal in Phnom Penh.
The court is currently hearing a case against the three most senior surviving Khmer Rouge members, accused of war crimes, genocide, and crimes against humanity.
Pol Pot, architect of the "Killing Fields", died in 1998.
Survivors hope the trial will help uncover truths and bring justice for their suffering under the Khmer Rouge.
(SOUNDBITE) (Khmer) SURVIVOR KEO NGA, 67 YEARS OLD, SAYING:
"I think this tribunal will not fail, but will go very slowly. Duch's response to the prosecutor's questions was slow and not clear."
The tribunal has been beset by controversy, as a second international judge resigned on Monday, citing interference by the Cambodian government in his investigation.
He was pursuing two unidentified suspects - believed to be former Khmer Rouge military commanders - for their role in the "year zero" revolution that killed as many as 2.2 million people between 1975 and 1979.
A monitor from the Open Society Justice Initiative says the trial has been derailed.
(SOUNDBITE) (English) TRIBUNAL MONITOR FROM OPEN SOCIETY JUSTICE INITIATIVE, CLAIR DUFFY, SAYING:
"This is supposed to be Cambodia's chance to see some justice for some of the worst atrocities committed last century, and actually it's just been hijacked by repeated political fighting over who the court will investigate and prosecute."
The current Cambodian Prime Minister Hun Sen is a former Khmer Rouge soldier who has previously said he would be happy if the war crimes tribunal packed up and left the country.
Nick Rowlands, Reuters.
