Speaking in front of hundreds of visitors, Khmer Rouge Tribunal chamber judge Nil Nonn announced the long-awaited verdict today in Cambodia's capital Phnom Penh.
The court sentenced them to life in prison.
Nuon Chea, who was the Khmer Rouge's chief ideologist and No. 2 leader, is seen on screen at the court's press center at the United Nations -backed war crimes tribunal on the outskirts of Phnom Penh, Cambodia, Friday, Nov. 16, 2018.
Nuon Chea was found guilty of genocide for attempting to wipe out Cham Muslims and ethnic Vietnamese Cambodians, while Khieu Samphan was found guilty of genocide against ethnic Vietnamese, and of various crimes against humanity.
The court found that during their rule, the Khmer Rouge had a policy to target Cham and Vietnamese people to create "an atheistic and homogenous society without class divisions", Judge Nil Nonn said in the verdict.
The killings cited under the law involve the Cham and Vietnamese ethnic groups.
"A great number of Cham civilians were taken. and were thus killed on a massive scale".
Vietnamese were subject to persecution, torture, and killing outside the notorious high-level prison as well.
He and his team would, Koppe said, not accept the conviction: "Everything that can be appealed will be appealed", he said.
Lawyers for Nuon Chea said they would appeal, and Khieu Samphan was expected to do the same.
Former Khmer Rouge Head of State Khieu Samphan in the courtroom at the ECCC in Phnom Penh
According to Friday's decision, Samphan and Chea, known as "Brother Number Two", were also found guilty of crimes against humanity, including murder, torture, imprisonment and forced marriage and rape in the the context of forced marriage, in which cadres compelled people to marry and have sexual intercourse against their will.
The following year on August 8, 2014, the two are found "guilty of crimes against humanity or extermination. political persecution and other inhumane acts" and jailed for life - a sentence that causes survivors to burst into applause as they weep after a 35-year wait for justice.
"Families of future spouses were not involved at all in the negotiation, communities were not included, tradition was absent from wedding ceremonies, and individuals agreed to get married for fear of being punished by the party", the judge added.
"The chamber finds that prisoners were brought to interrogation rooms, handcuffed and blindfolded, their legs chained during questioning", the Extraordinary Chambers in the Courts of Cambodia said in its verdict, The Guardian reported.
Up to two million people are believed to have died under the brief but systematically brutal Khmer Rouge regime between 1975 and 1979.
Like Khieu Samphan, who claimed to have no knowledge of mass killings when he was sentenced on earlier charges in June previous year, Nuon Chea also claimed to be unaware of the mass killings being carried out, a claim that was debunked during court proceedings.
Nicholas Koumjian, global co-prosecutor, told a news conference after the verdict was read out on Friday that the decision marked an important day for worldwide justice and Cambodia.
"I certainly wish the tribunal would move faster... In short, I am innocent in relation to those allegations", he said.
Worldwide judges want the trial to proceed while the Cambodian judges reportedly do not.
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And last week, over 20 individuals on the list told Reuters they would refuse to return to Rakhine, as they were terrified. Another community leader, Dil Mohammad, said the situation in Myanmar was not yet conducive for the refugees' return.