The trial of the three top surviving Khmer Rouge leaders accused of orchestrating Cambodia's "killing fields" in the late 1970s opened Monday in the capital, Phnom Penh.
The men, in poor health and aged in their eighties, face charges if genocide and crimes against humanity.
Among the accused is Nuon Chea, 85, also known as Brother Number Two, reported the BBC, adding that he was the "right-hand man" of supreme leader Pol Pot, who died in 1998 while a prisoner of his own comrades.
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Also on trial are the regime's former head-of-state Khieu Samphan, 80, and former foreign minister Ieng Sary, 86.
Former social affairs minister Ieng Thirith, 79, who is married to Sary, was declared unfit to go on trial because she has Alzheimer's disease. She was the regime's minister for social affairs.
The process of trying senior Khmer Rouge leaders comes more than three decades after the Khmer Rouge regime fell in 1979.
The UN-backed trial began hearing four days of opening statements, with court spokesman Lars Olsen describing the opening day as a "major milestone".
The charges against the accused include crimes against humanity, genocide, religious persecution, homicide and torture, the Associated Press reported.
All three deny the charges.
Anne Heindel, legal adviser to the independent Documentation Center of Cambodia, which collects evidence of Khmer Rouge atrocities, said the trial marks the first time the Khmer Rouge leadership are being made responsible for causing the deaths of nearly 2 million people, the AP reported.
"There is hope that it will help Cambodians understand why it happened, why Khmer killed Khmer, and will teach the younger generation to ensure it will never happen again."
During the Khmer Rouge's rule, from 1975 to 1979, an estimated 1.7 million people died from execution, starvation, exhaustion or lack of medical care.