A United Nations-backed tribunal in Cambodia has charged Khieu Samphan, the former head of state for the Khmer Rouge, with genocide.
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.
Two other Khmer Rouge leaders, Ieng Sary and Nuon Chea, have also been charged with genocide.
All charges relate to the men's treatment of Cambodia's Vietnamese and Muslim minorities, the BBC reports.
The men had already been charged with war crimes and crimes against humanity and have been detained.
The trial is not expected to begin before 2011.