"It is easy to say that I should have known everything, I should have understood everything, and thus I could have intervened or rectified the situation at the time," Khieu Samphan defiantly told the court. "Do you really think that that was what I wanted to happen to my people?"
"The reality was that I did not have any power," he said.
Nuon Chea also defended his actions, saying he never ordered Khmer Rouge cadres "to mistreat or kill people to deprive them of food or commit any genocide."
Unlike Khieu Samphan, however, Nuon Chea accepted "moral responsibility" for the deaths, repeating previous efforts to distance himself from actual crimes.
"I would like to sincerely apologize to the public, the victims, the families, and all Cambodian people," said the frail former leader, speaking steadily as he read from pages of notes. "I wish to show my remorse and pray for the lost souls that occurred by any means" during the Khmer Rouge rule.
Nuon Chea's words are unlikely to be any consolation for survivors, hundreds of whom crowded the courtroom and the tribunal's grounds.
"He is just trying to cheat the court so that he can be freed," said Bin Siv Lang, a 56-year-old woman who lost 11 relatives during the Khmer Rouge rule. "If he issued no orders to kill people, his subordinates would not have killed."
Death and disability have robbed the tribunal of other defendants. Khmer Rouge Foreign Minister Ieng Sary died in March, and his wife Ieng Thirith, the regime's social affairs minister, was declared unfit for trial in September 2012 after being diagnosed with dementia. The group's top leader, Pol Pot, died in 1998.
The tribunal, launched in 2006, so far has convicted only one defendant, Khmer Rouge prison director Kaing Guek Eav, who was sentenced to life imprisonment in 2011.
The present trial's focus is on the forced movement of people and excludes some of the gravest charges related to genocide, detention centers and killings.
Nuon Chea said he believed his trial proved he "was not engaged in any commission of the crimes as alleged by the co-prosecutors. ... In short, I am innocent."
Khieu Samphan said bitterly that he had lost faith in the tribunal because "no matter how hard I try to explain, they (the court's judges) will only turn their deaf ears at me."
"It is clear that everyone wants only one thing from me that is, my admission of guilt ... concerning the acts that I have never ever committed at all."
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Associated Press writer Sopheng Cheang contributed to this report.