The only two surviving leaders of Cambodia's Khmer Rouge regime have been sentenced to life imprisonment after being found guilty of crimes against humanity.

Khieu Samphan, the 83-year-old former head of state, and Nuon Chea, 88, deputy to the dictator Pol Pot, were sentenced over alleged crimes committed in the 1970s in which up to two million Cambodians are believed to have died.

The two octogenarians were leading exponents of the totalitarian fantasy: Year Zero.

The idea behind Year Zero was that the existing society within Cambodia had to be completely destroyed or discarded and a new revolutionary culture had to replace it from scratch. All history of the nation Year Zero was deemed largely irrelevant and had to be purged and replaced from the ground up.

In Cambodia, teachers, artists, and intellectuals were especially singled out and executed during the purges accompanying Year Zero. Millions were forced out of the cities to create a agaranian utopia, of whom thousands subsequently starved.

The tribunal's chief judge, Nil Nonn, asked both men to rise for the verdicts but the frail Nuon Chea, wearing dark sunglasses, said he was too weak to stand from his wheelchair and was allowed to remain seated.

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Former Khmer Rouge head of state Khieu Samphan is pictured at the Extraordinary Chambers in the Courts of Cambodia on 22 November 2011Getty
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Nuon Chea, who was deputy to leader Pol Pot, is pictured at the Extraordinary Chambers in the Courts of Cambodia on 22 November 2011Getty

Nearly a quarter of the population — about 1.7 million people — died under rule of the Khmer Rouge through a combination starvation, medical neglect, overwork and execution when the group held power in 1975-79.

Survivors of the regime travelled from across the country to witness the historic day, filling the several hundred seats available to the public at the tribunal.

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Soum Rithy, who lost his father and three siblings during the Khmer Rouge regime, hugs another survivor after the verdict was delivered in the trial of former head of state Khieu Samphan and former Khmer Rouge leader 'Brother Number Two' Nuon Chea at the Extraordinary Chambers in the Courts of CambodiaReuters

Many have criticised the slow justice, however, and its cost. The tribunal, formally known as the Extraordinary Chambers in the Courts of Cambodia, began operations in 2006. It has since spent around £120 million.

The current trial began in 2011, with four senior Khmer Rouge leaders. Only two remain. Former Foreign Minister Ieng Sary died in 2013, while his wife, Social Affairs Minister Ieng Thirith, was deemed unfit to stand trial due to dementia in 2012. The group's top leader, Pol Pot, died in 1998.

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Former foreign minister Ieng Sary and his wife and ex-social affairs minister Ieng ThirithGetty
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Former Khmer Rouge leader Pol Pot talks to a journalist in the guerrilla's jungle hide-out in Northern Cambodia, reportedly on 4 January 1998. The photographer claims that his camera was wrongly set and that the date showing is erroneous. Pol Pot died on 15 April 1998.AFP

Khieu Samphan has acknowledged that mass killings took place. But testifying before the court in 2011, he claimed he was just a figurehead who had no real authority. He denied ordering any executions himself, calling the allegations a "fairy tale". Instead, he blamed Pol Pot for its extreme policies.

Nuon Chea, who is known as Brother No 2 for being Pol Pot's trusted deputy, had also denied responsibility, testifying in 2011 that Vietnamese forces — not the Khmer Rouge — had killed Cambodians en masse. "I don't want them to believe the Khmer Rouge are bad people, are criminals," he said of those observing to the trial. "Nothing is true about that."

Both men now face a second trial, reportedly due to start in either September or October, this time on charges of genocide. The trial is expected to take years to complete.

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A visitor to the Genocide Museum housed in the former Tuol Sleng prison looks at photos of men killed during the Khmer Rouge regimeGetty
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A woman looks at a wall of photographs of prisoners of the Khmer Rouge regime at Tuol Sleng prisonGetty
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A tourist takes pictures of cells at the former Khmer Rouge Tuol Sleng prisonReuters
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A photograph hangs on the wall in a room once used as a torture chamber at the former Khmer Rouge Tuol Sleng prisonReuters
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Khmer Rouge instruments of torture are displayed at Tuol Sleng prison, now the Genocide MuseumReuters
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Skulls are placed behind glass at a memorial made with the bones of more than 8,000 victims of the Khmer Rouge regime at Choeung Ek, a 'Killing Fields' site located on the outskirts of Phnom PenhReuters

The Khmer Rouge captured Phnom Penh on 17 April 1975 after a siege. They carried out a radical programme of reforms that included confiscating all private property and relocating people from cities to collective farms. Families were split up, schools were outlawed, currency was abolished and forced labour was widespread.

Every Cambodian was forced to become a farmer, but as the city dwellers had no agricultural knowledge, famine was inevitable.

The Khmer Rouge executed huge numbers of people: those they suspected of being spies, anyone carrying out any religious observance, and anyone they thought might be intellectuals – such as people wearing glasses.

The Khmer Rouge regime fell in 1979, after a failed invasion of Vietnam. Between 1.4 and 2.2 million people died of execution, starvation, exhaustion or lack of medical care during the Khmer rouge regime.

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17 April 1975: Khmer Rouge fighters celebrate as they enter Phnom Penh, the capital of CambodiaAFP
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17 April 1975: A Khmer Rouge guerrilla holding a gun rides a motorcycle into Phnom Penh on the day Cambodia fell under the control of the Communist Khmer Rouge forces. The Cambodian capital surrendered after a three and a half-month siege.AFP
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17 April 1975: A woman cries over a dead body after the Khmer Rouge entered the Cambodian capital.AFP
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Cambodian civilians and Vietnamese soldiers open mass graves.Getty
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Left: An undated file picture of Pol Pot, probably taken in 1989 in western Cambodia. Right: Former top members of the Khmer Rouge: prime minister Khieu Samphan (top-L), deputy Nuon Chea (top-R), army chief of staff Ta Mok (bottom-L) and former foreign minister Ieng Sary (bottom-R).AFP
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27 November 1991: Khieu Samphan, then leader of the Khmer Rouge, is pictured wearing an army helmet and a pair of underpants as a bandage after he was clubbed by an angry mob in Phnom PenhAFP
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17 April 1998: A Khmer Rouge soldier puts a block of ice on the body of former leader Pol Pot prior to his cremationAFP