Nuon Chea, 92, was considered the Khmer Rouge's main ideologist and Pol Pot's right-hand man, while Khieu Samphan, 87, served as the head of state, presenting a moderate veneer as the public face for the highly secretive group. The prison and the courthouse were custom built for the use of the tribunal, which is officially called the Extraordinary Chambers in the Courts of Cambodia, or ECCC.
A United Nations -backed court on Friday convicted two Khmer Rouge leaders of committing genocide, the first of Pol Pot's cadre to be found guilty for targeting minority groups for elimination during the regime's rule in Cambodia in the 1970s.
An worldwide tribunal in Cambodia on November 16 convicted two men for their part: the head of state, Khieu Samphan, 87, and leader Pol Pot's deputy, Nuon Chea, 92, known as "Number Two".
"Today, I am very happy that the both Khmer Rouge leaders were sentenced to life in prison".
Some 1.7 million people died from starvation, torture, execution and forced labour under the Khmer Rouge.
But the BBC's South East Asia correspondent Jonathan Head says the larger-scale killings of the Cambodian population do not fit the narrow global definition of genocide, and have been prosecuted instead as crimes against humanity.
Khieu Samphan described the claim of genocide as "Vietnamese propaganda", a defence he and other former Khmer Rouge leaders have made previously.
"They brought suffering to my relatives" 72-year-old Cham Muslim Los Sat, who lost many family members, told the AFP news agency at the court".
They sought to create a self-reliant, agrarian society: cities were emptied and residents forced to work on rural co-operatives.
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The U.N. has said it should be investigated as a genocide, but the US has stopped short of that and called it ethnic cleansing. Small-scale demonstrations have taken place in northern Rakhine State in protest against the prospect of a mass return.
The number of allegations against Nuon Chea and Khieu Samphan was so vast the court split the trials into a series of smaller hearings in 2011.
The Khmer Rouge came to power in the instability that swept through Southeast Asia in the wake of the Vietnam War.
Nuon Chea suffers heart problems and was allowed to move from the hearing room to a separate holding room.
The tribunal in 2010 also convicted Kaing Guek Eav, known as Duch, who as head of the Khmer Rouge prison system ran the infamous Tuol Sleng torture centre in Phnom Penh.
Many believe the decision will be the last for the tribunal, which has been marred by allegations of political interference.
Hun Sen himself was a midlevel Khmer Rouge commander before defecting while the group was still in power, and several senior members of his ruling Cambodian People's Party share similar backgrounds.
It abolished money, free markets, normal schooling, private property, foreign clothing styles, religious practices, and traditional Khmer culture, according to the Cambodia Tribunal.
The court, a hybrid UN-Cambodian tribunal, was set up in 2005 to bring to justice "those most responsible" for the deaths under the Khmer Rouge, has convicted just three people. "They will always be political and fall short of expectations", Alexander Hinton, an anthropology professor at Rutgers University and author of two books about the tribunal, said ahead of Friday's verdicts.