Kaing Guek Eav, also known as "Duch" and the ex-commandant of the notorious S-21 prison, sat impassively in court as a judge made opening statements.
He had been regretful on the eve of his trial by the "Killing Fields" court set up to prosecute "those most responsible" for the 1975-79 reign of terror, one of the darkest chapters of the 20th century.
"He said to the victims, I ask your forgiveness, I ask your forgiveness," French lawyer Francois Roux told Reuters Television after visiting his client for two hours at a detention centre near the specially built court outside the Cambodian capital.
Hundreds of victims of Khmer Rouge atrocities were lining up on Tuesday to get into court, but the proceedings would be mostly procedural with the main trial due to start in March and a verdict expected by September.
"I came here to see the trial with my own eyes, so I could tell villagers who could not be here," Mahd Musa, 54, who lost seven family members in the Khmer Rouge era, told Reuters.
"It is a very important day for me. I cannot miss it."
The hearing marks a turning point for the strife-torn country where nearly every family lost someone during the Khmer Rouge era. The trial also ends a decade of delays at the Cambodian-UN tribunal due to wrangling over jurisdiction and cash.
Advocates hope the tribunal – formerly known as the Extraordinary Chambers in the Courts of Cambodia (ECCC) – will serve as a model of professionalism for the country's erratic and politicised judiciary.
Critics say the tribunal's integrity is threatened by allegations of corruption and political interference, particularly on the issue of pursuing other Khmer Rouge suspects.
Duch also faces charges of war crimes, torture and homicide while chief of S-21, where at least 14,000 enemies of the revolution were jailed and later killed.
The greying 66-year-old is one of five ageing senior cadres charged for their roles in Pol Pot's "Year Zero" revolution to achieve an agrarian utopia.
He is expected to be a key witness in the future trials of "Brother Number Two" Nuon Chea, the regime's ex-president Khieu Samphan and Ieng Sary, its foreign minister, and his wife.
The four others have denied knowledge of any atrocities by the Khmer Rouge during its rule, which began by driving everyone out of the cities with whatever they could carry.
If convicted, the five could face life in prison.
‘FULL OF BLOOD’
A bid to go after other suspects was brushed aside last month by the tribunal's Cambodian co-prosecutor, who said it would not help national reconciliation. The government denied any meddling, but rights groups are concerned.
"Any hint of political manipulation at the tribunal will undermine its credibility with the Cambodian people," said Sara Colm, Cambodia-based senior researcher at Human Rights Watch.
Some 300 local and foreign journalists are accredited for the first day of the trial to be televised to a potential audience of millions in Cambodia.
A born-again Christian, Duch has confessed in interviews with Western reporters that he committed multiple atrocities as head of the infamous Tuol Sleng, or S-21, interrogation centre.
Most victims were tortured and forced to confess to a variety of crimes – mainly being CIA spies – before being bludgeoned to death in a field on the outskirts of the city.
Women and children were also killed. Only a few survived.
"Duch's hands are full of blood. It's time for Duch to pay for his actions," said 39-year-old Norng Chan Phal, a child survivor whose mother was killed at S-21 months before Vietnamese soldiers toppled Pol Pot's regime in 1979.
Pol Pot's death in 1998 was followed by a formal Khmer Rouge surrender which helped to usher in a decade of peace and stability, threatened now by the global economic downturn.