The defendants include the "number two" in Pol Pot's regime, Nuon Chea. They face charges including genocide and crimes against humanity over the deaths of up to 2m Cambodians in 1975-79.

They all deny the accusations, and the trial is likely to last for years.

Last July, former Khmer Rouge member Kaing Guek Eav, known as Comrade Duch, was jailed for 35 years.

But because of time already served and compensation for a period of illegal detention, Duch - the former head of a notorious prison where some 15,000 died - will be free in 19 years.

Led by Pol Pot, who died in 1998, the Maoist Khmer Rouge regime was overthrown in 1979.