Khieu Samphan, detained in November 2007, denies responsibility
A UN-backed tribunal in Cambodia has charged Khieu Samphan, formerly the head of state for the Khmer Rouge, with genocide.
The move came after genocide charges were filed against two other Khmer Rouge leaders, Ieng Sary and Nuon Chea.
All the genocide charges relate to the men's treatment of Cambodia's Vietnamese and Muslim minorities.
All three men had already been charged with war crimes and crimes against humanity.
Those charged are already in pre-trial detention although the trial is not expected to begin before 2011.
Up to two million people are thought to have died under the Khmer Rouge's rule.
Khieu Samphan, 78, has never denied these deaths, but both he and his lawyers insist that, as head of state, he was never directly responsible.
One member of his defence team is the infamous French lawyer Jacques Verges, whose previous clients have included Nazi war criminal Klaus Barbie and Venezuelan hijacker Carlos the Jackal.
Mr Verges, 83, has known Khieu Samphan since they were both involved in left-wing student activities in France in the 1950s.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Up to two million people thought to have died from starvation, overwork or execution
He says he has lived a life of poverty since the Khmer Rouge regime was toppled.
A court official confirmed that the allegations related to the treatment of two minority groups: Cham Muslims and ethnic Vietnamese people.
Researchers believe that the Khmer Rouge killed hundreds of thousands of Chams because of their religious beliefs.
The accusation of genocide carries enormous symbolic weight, says the BBC's Guy De Launey in Cambodia's capital, Phnom Penh.
Final arguments were heard last month in the trial of Khmer Rouge prison chief Kaing Guek Eav, known as Comrade Duch, who has admitted being responsible for overseeing the deaths of 15,000 people.
Judges at the tribunal are expected to rule on his verdict early next year.