The former chief of the Khmer Rouge’s notorious S-21 prison yesterday delved into details of the party’s internal purges and the regime’s final days.

According to ex-warden Kaing Guek Eav, better known as Duch, Khmer Rouge cadre were arrested “en masse” on the day Sao Phim – the East Zone secretary who later killed himself at S-21 – was arrested.

In the days prior, Duch received instructions from his superiors to make room for an influx of new prisoners, meaning large numbers of inmates were carted off to the Choeung Ek killing fields.

According to Duch, on one occasion there were 200 or 300 prisoners from the east who were brought in on trucks and immediately sent to their deaths. When prosecutor Dale Lysak asked who had ordered those people to be executed without interrogation, Duch responded: “It was brother Nuon”.

Nuon Chea, former second in command to Khmer Rouge leader Pol Pot, is on trial for crimes against humanity alongside former head of state Khieu Samphan. Chea’s defence has strenuously denied his involvement in the running of S-21.

Duch recalled another en masse removal of prisoners from S-21, in early January 1979, just days before the Vietnamese forces claimed control of Cambodia.

During a meeting with a handful of cadre in Phnom Penh led by Samphan and attended by Duch, the head of state was calm and diverted attention away from the imminent threat during the last days before the regime toppled.

“He said the situation was normal . . . we were told to proceed with our normal work,” Duch said.

Duch, who is serving a life sentence after his own trial, recalled an encounter with his former comrade Vorn Vet – a member of the Khmer Rouge inner circle who was purged in 1978. Duch said Vet had described Pol Pot as a “two-faced” person plagued by “paranoia”.

Duch also recalled receiving a call from Nuon Chea in late 1978 calling for an interim halt to torture – such as beatings and electrocution – during interrogations.

“For Khmer prisoners, there was no reason to beat them, and we could retain 70 per cent of the detailed confessions . . . As for the Yuon [an often derogatory term for the Vietnamese], the practice remained the same,” Duch said. “Sometimes brother Nuon poked his fingers deep into the business of S-21.”