The two most senior surviving Khmer Rouge leaders were sentenced to life in prison Thursday by Cambodia's UN-backed tribunal — only the second verdict in nearly a decade of investigations into the regime's responsibility for the deaths of nearly a quarter of all Cambodians during the late 1970s.

Nuon Chea, 88, who served as prime minister and was "Brother Number Two" to Khmer Rouge leader Pol Pot, and former president Khieu Samphan, 83, were found guilty by the court of crimes against humanity.

The charges relate to the Khmer Rouge's disastrous campaign to ruralize Cambodian society, when they forcibly emptied cities and marched citizens to the countryside. Khmer Rouge leaders "considered city-dwellers to be intrinsically disloyal and concluded that they would remain politically and ideologically corrupt," the court recounted.

The Extraordinary Chambers of the Courts of Cambodia, as it is officially known, found the two men had engaged in a "joint criminal enterprise" that caused a "widespread and systematic attack against the civilian population of Cambodia."

Both men plan to appeal, yet the court ruled they would be remanded to prison during that process.

After winning a five-year civil war in 1975, the regime first targeted officials from the previous government, but soon turned their erratic wrath on intellectuals and minorities, people who wore glass or spoke foreign languages, even those who cried at funerals.

Between 1975 and 1979, an estimated 1.7 million Cambodians died from hunger, disease, overwork or were murdered by the regime.

Within hours of entering Phnom Penh on April 17, 1975, Khmer Rouge leaders began to order city residents to leave immediately. The court estimated at least two million people, nearly the entire population of the capital, was "forcibly transferred, often at gunpoint, with almost no prior warning and in terrifying and violent circumstances."

The trial held 20 months of evidentiary hearings that included testimony from victims and over 1,000 written statements from witnesses and survivors.

On Thursday, the verdict was met with cheers and sobs as victims and their relatives in the courtroom embraced one another.

"Now that the high-ranking officials of the Khmer Rouge regime have been found guilty, they will finally be able to mourn," Karine Bonneau, head of the International Justice Desk at the International Federal for Human Rights, who works with the ECCC, told VICE News. "The verdict is a recognition for them of what has happened and of them as victims."

Charges against Nuon Chea and Khieu Samphan were split into two trials, partly in an effort to see the two men survive to see part of the proceedings conclude before they die.

Due to health concerns, Nuon Chea was at times allowed to provide testimony from a bed in the basement of the tribunal. At Thursday's proceedings, he claimed he was too weak to rise as the verdict was read.

'I personally want triple life sentences for them.'

Pol Pot died in 1998, shortly after the remaining Khmer Rouge holdouts began contemplating turning him over the authorities.

Hearings for the second portion of Nuon Chea and Khieu Samphan's trial, charging the two men with genocide for the deaths of an estimated 100,000-500,000 ethnic Chan Muslims and 20,000 Vietnamese, as well as rapes that resulted from forced marriages, began last week.

Prior to Thursday, the tribunal had only convicted one other Khmer Rouge official, Kaing Guek Eav, known as "Duch," since it was opened in 2006. Duch was found guilty in 2010 and sentenced to 35 years for overseeing the state's security apparatus and network of prison camps.

For some Cambodians, the sheer scale of the Khmer Rouge's crimes can never be encapsulated in a court room.

"This is not a victory," Youk Chhang, executive director of the Documentation Center of Cambodia, an organization that catalogues the crimes of the Khmer Rouge, and one of the those forced to leave Phnom Penh in 1975, told VICE News. "The verdict can only build hope and continuity to search for truth which is important for the future. But the verdict cannot bring back the two million Cambodians who died."

"I personally want triple life sentences for them," he added, before echoing a common concern that the men will enjoy relative luxury in the tribunal's prison while their appeal proceeds.

The $200 million court — mostly funded by foreign donors and the UN — has come under criticism for its lethargic investigations, allegations of corruption, and lack of convictions.

Critics also expressed concern about the ordering of the split trials of Nuon Chea and Khieu Samphan, with the most severe charge of genocide and ethnic cleansing pushed into proceedings that will likely last until 2016.

"The sequencing approach may not have accelerated the entire proceedings," said Bonneau, though she added the first trial lay "a general foundation for all allegations against the accused."

Cambodian president Hun Sen has attempted to meddle in the tribunal's affairs, declaring definitively that the second trial of Nuon Chea and Khieu Samphan should be its last.

Hun Sen was an officer in the Khmer Rouge before he fled the country as the regime's purges turned inward. Many bureaucrats and local officials, as well as members of the military are holdovers from that period.

Two further cases, for now only in their investigative phases, are widely believed to implicate officials close to Hun Sen. It's unclear if those will proceed.

Wan-Hea Lee, Representative at the UN's Human Rights division in Phnom Penh, told VICE News that after three years, the result was a positive first step.

"The verdict demonstrates that perseverance can pay off and that justice is a common human value, and gives hope that the worst crime — genocide — will also be duly tried in the subsequent cases."

Despite outreach efforts by the court, many Cambodians born after 1979 have viewed the trials with ambivalence. Cambodia is rapidly modernizing, especially in Phnom Penh, where in the past 18 months once rare smart phones have become ubiquitous. Young Cambodians have taken to social networks like Twitter to discuss the trials, but also to talk about the same things young people do the world over.

"It is a strong and positive message for younger generations that what happened constitute crimes against humanity, and that these crimes cannot go unpunished," said Bonneau. "Victims of the Khmer Rouge regime have been waiting for almost 40 years."

Follow Samuel Oakford on Twitter: @samueloakford