Three top Khmer Rouge leaders went on trial in Cambodia Monday on charges of genocide and other atrocities during the communist movement's reign of terror in the late 1970s.

Nil Nonn, the judge presiding over the U.N.-backed court, declared the trial open and read out the names of the defendants.

Khmer Rouge head of state Khieu Samphan, reputed chief ideologue Nuon Chea and Khmer Rouge Foreign Minister Ieng Sary listened silently as the charges were read aloud in a crowded courtroom in Phnom Penh.

The three are accused of crimes against humanity, including genocide, homicide and torture. The Khmer Rouge ruled from 1975 to 1979, when up to two million Cambodians – one quarter of the population – are estimated to have died.

A fourth defendant, Ieng Thirith, was ruled unfit to stand trial last week because she has Alzheimer's disease. She is Ieng Sary's wife and served as the minister for social affairs for the Khmer Rouge.

The Khmer Rouge's supreme leader Pol Pot died in 1998 in a jungle camp, where he was held prisoner after his former comrades turned on him.