Cambodia's U.N.-backed war crimes tribunal has opened the trial of the top four surviving leaders of the brutal Khmer Rouge regime, accused of genocide and other atrocities in the late 1970s.

The defendants are Khieu Samphan, 79, the nominal head of state; Nuon Chea, 84, described as the Khmer Rouge's ideologue; Ieng Sary, 85, the foreign minister; and his wife, Ieng Thirith, 79, who was minister of social affairs. All have denied the charges against them.

The day's proceedings were devoted to procedural issues, with the presentation of evidence and testimony not expected to begin for several weeks. The trial is expected to last for years, raising concerns about whether the elderly defendants all will live to see it end.

In its only previous case, the court last year convicted Kaing Guek Eav, who ran the main prison and torture house of the Khmer Rouge. He was given a term of 35 years, which was reduced to 19 years.

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