The four most senior survivors from Cambodia's notorious Khmer Rouge regime have gone on trial for war crimes allegedly committed during the hardline communist movement's four-year reign of terror in the late 1970s.

The defendants, now elderly and frail, appeared in a Phnom Penh courtroom Monday, at the beginning of their long-awaited trial. All four deny the charges, which include genocide, religious persecution and torture linked to the deaths of as many as 2 million people between 1975 and 1979.

On trial are Khieu Samphan, 79, the nominal Khmer Rouge head of state; Nuon Chea, 84, described as the regime's chief ideologue; Ieng Sary, 85, the foreign minister; and his wife, Ieng Thirith, 79, who was minister of social affairs. Khmer Rouge leader Pol Pot died in 1998.

The trial, which analysts say could take years, is the showcase event for the United Nations-backed war crimes tribunal, which was created to demonstrate impartial justice and foster national healing.

About 500 people, many of them victims of the Khmer Rouge, filled the courtroom Monday. The four defendants sat together without handcuffs behind a curtain to hear the charges read against them. Three were later excused from the hearing on health grounds.

Proceedings Monday focused on moves by lawyers for Ieng Sary to have his case thrown out on grounds of double jeopardy. He was sentenced to death in absentia in 1979 in an unsanctioned trial conducted by Vietnamese invaders, and later pardoned by then-Cambodian King Norodom Sihanouk. Defense lawyers argued Monday their client should not be tried twice for the same crimes.

Proceedings over the next four days are also expected to focus on procedural matters, with the presentation of evidence and testimony not expected to begin for several weeks. The trial is expected to involve hundreds of witnesses, raising concerns about whether all of the elderly defendants will live to see it end.

In its only previous case, the court last year convicted Kaing Guek Eav, better known as Duch, who ran the main prison and torture house of the Khmer Rouge. He was sentenced to 35 years in prison, which was reduced to 19 years.