Comrade Duch, a former senior figure of the Khmer Rouge guerrila group convicted of crimes against humanity in Cambodia, has died.
He was serving a life sentence after being sentenced by a UN-backed court.
Kaing Guek Eav, known as Comrade Duch, ran the notorious Tuol Sleng prison where thousands of people were tortured and murdered in the late 1970s.
As many as two million people are believed to have died under the Khmer Rouge, a Maoist regime that controlled Cambodia from 1975 to 1979.
In 2010, Duch became the first senior Khmer Rouge leader to be convicted by a UN-backed tribunal set up in the Cambodian capital Phnom Penh.
He was sentenced in 2012.
He died on Wednesday, aged 77, a spokesperson for the tribunal said. He had been ill for many years.
Comrade Duch ran Phnom Penh’s S-21 prison, also known as Tuol Sleng, the most notorious torture site during the Khmer Rouge regime.
It is thought that at least 15,000 men, women and children deemed enemies of the regime passed through the gates of the former school-turned-prison.
Most of them were tortured, forced to confess to fictitious crimes against the Khmer Rouge and then put to death at the so-called “killing fields” just outside the capital.