The pair are already serving life sentences. Photo: Reuters

Khieu Samphan was in court to hear the verdict. Photo: Reuters

Victims and their relatives were in the courtroom to see the ruling read out. Photo: GETTY IMAGES

Nuon Chea, 92, was the deputy of regime leader Pol Pot, and Khieu Samphan, 87, was head of state.

They were on trial at the UN-backed tribunal on charges of exterminating Cham Muslims and ethnic Vietnamese.

The guilty verdict is the first official ruling that what the regime did was genocide, as defined under international law.

The pair were also found guilty of a litany of other crimes, including the crimes against humanity of murder, extermination, enslavement and torture.

Up to two million people are believed to have died under the brief but systematically brutal Khmer Rouge regime between 1975 and 1979.

Most were from the Khmer majority and so the larger-scale killings of the Cambodian population do not fit the narrow international definition of genocide, and have been instead prosecuted instead as crimes against humanity, says BBC South East Asia correspondent Jonathan Head.

These verdicts will almost certainly be the last from an unusual attempt at transnational justice that has lasted more than a decade, our correspondent adds.

Life Sentences 

The two men - already serving life sentences for crimes against humanity - have again been sentenced to life.

They are two of only three people ever convicted by the tribunal, which has faced criticism for its slow pace and for being subject to alleged political interference.

Judge Nil Nonn read out the lengthy and much-anticipated ruling to a courtroom in Phnom Penh full of people who suffered under the Khmer Rouge.

He described the terror of the regime, and spoke of forced marriages where couples were ordered to have children.

But the landmark moment came when Nuon Chea was found guilty of genocide for the attempt to wipe out Cham Muslims and ethnic Vietnamese Cambodians, and Khieu Samphan was found guilty of genocide against the ethnic Vietnamese.

Researchers estimate that 36% of the Cham population of 300,000 died under the Khmer Rouge. Most of the Vietnamese community were deported, and the 20,000 who remained were all killed.

Why is the genocide verdict significant?

The Khmer Rouge's crimes have long been referred to as the "Cambodian genocide", but academics and journalists have debated for years as to whether what they did amounts to that crime.

The UN Convention on Genocide speaks of "intent to destroy, in whole or in part, a national, ethnic, racial or religious group".

So prosecutors at the tribunal tried to prove that the Khmer Rouge specifically tried to do that to these groups - something some experts, including Pol Pot biographer Philip Short, say they did not.

During the trial, a 1978 speech from Pol Pot was cited in which he said that there was "not one seed" of Vietnamese to be found in Cambodia. And historians say that indeed a community of a few hundred thousand was reduced to zero by deportations or killings.

Apart from being targeted in mass executions, Cham victims have said they were banned from following their religion and forced to eat pork under the regime.

The verdict today may not end the debate completely, but victims groups have long waited for this symbol of justice.

"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. " I am really satisfied with the sentences."