The top surviving leaders of the Khmer Rouge appeared in a Cambodian courtroom Monday at the beginning of their long-awaited trial for crimes against humanity.

The trial is the showcase event for the United Nations-backed war crimes tribunal, which was created to demonstrate impartial justice and foster healing in a country long divided by the Khmer Rouge atrocities in the late 1970s.

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

On trial are Khieu Samphan, 79, the nominal head of state; Nuon Chea, 84, described as the Khmer Rouge's chief ideologue; Ieng Sary, 85, the foreign minister; and his wife, Ieng Thirith, 79, who was minister of social affairs. All have denied the charges, which include war crimes, genocide, religious persecution and torture.

Pol Pot, the Khmer Rouge leader, died in 1998.

Proceedings over the next four days will be devoted to procedural issues, with the presentation of evidence and testimony not expected to begin for several weeks. The trial is expected to involve hundreds of witnesses and last for years, 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.

As many as 2 million people are believed to have been executed or died from starvation and overwork under the Khmer Rouge regime.