Former Khmer Rouge prison chief Kaing Guek Eav, also known as Duch, has been taken to one of Cambodia's killing fields to re-enact his alleged actions.
Duch (centre) ran the notorious Tuol Sleng jail
The visit to Choeung Ek, where some 16,000 people were buried after being tortured, is part of investigations by the UN-backed genocide tribunal.
Duch is the first of five senior Khmer Rouge officials to be charged.
The Khmer Rouge regime ruled Cambodia between 1975 and 1979 and is blamed for more than one million deaths.
Duch, who is now in his 60s, ran the notorious Tuol Sleng jail in Phnom Penh under the Khmer Rouge, and is accused of overseeing the torture and killing of inmates.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, (above) who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Brutal regime that did not tolerate dissent
More than a million people thought to have died from starvation, overwork or execution
He was driven to Choeung Ek, 10km (six miles) south of the capital Phnom Penh, in a heavily guarded convoy.
The re-enactment was closed to the public and the media.
It is not known if any survivors were witnessing Duch's version of what happened.
Duch was due to go through a similar re-enactment at the former Tuol Sleng jail - now a genocide museum - on Wednesday.
He was arrested and detained in July 2007. In December, a court dismissed an appeal for bail by his lawyers who argued that he was held without charge under the jurisdiction of another court for eight years.
Choeung Ek serves a reminder of the atrocities to young Cambodians
A date for the trial has yet to be set.
Five senior Khmer Rouge officials are now in the custody of the tribunal.
Those also facing charges include Nuon Chea, second-in-command of the late Khmer Rouge leader Pol Pot, the former foreign and social affairs ministers Ieng Sary and Ieng Thirith, and former head of state Khieu Samphan.
Pol Pot died in 1998.