Now old and infirm, the four top surviving leaders of Cambodia's brutal Khmer Rouge regime went on trial before a UN-backed court for the first time Monday, facing justice three decades after their plan for a communist utopia left an estimated 1.7 million people dead.

Security was tight at the tribunal, with dozens of police on guard and 500 spectators — the majority victims of the 1975-79 regime — watching from the gallery.

With Khmer Rouge leader Pol Pot long dead, this may be the nation's best chance to hold the accused architects of the "killing fields" and the enslavement of millions of Cambodians accountable, though all four say they are innocent. Nuon Chea, centre, who was Pol Pot's No. 2 and the chief ideologist of the Khmer Rouge, sits during the second trial of the top leaders in a UN-backed war crimes tribunal. Extraordinary Chambers in the Courts of Cambodia, Mark Peters/Associated Press

On trial Monday are 84-year-old Nuon Chea, who was Pol Pot's No. 2 and the group's chief ideologist; 79-year-old former head of state Khieu Samphan; ex-foreign minister Ieng Sary, 85; and his 79-year-old wife, Ieng Thirith, who served as minister for social affairs.

Accused sat side by side

The tribunal's chief judge, Nil Nonn, opened the court session and was expected to read out charges against the four that included crimes against humanity, war crimes, genocide, religious persecution, homicide and torture.

The four accused sat side by side, without handcuffs, their faces obscured behind a curtain.

File photos of the four top surviving leaders of the Khmer Rouge regime, from left to right: Nuon Chea, Khieu Samphan, Ieng Sary, and his wife, Ieng Thirith. The four are accused of helping to orchestrate the enslavement and killings of millions of Cambodians three decades ago. Heng Sinith/Chor Sokunthea, file/Associated Press

Together, they form what the tribunal calls Case 002. The chief jailer of a notorious Khmer Rouge prison was convicted last year in the breakthrough Case 001. Political and financial pressures on the tribunal are raising doubts over whether there will ever be a Case 003.

Although this week's court sessions will be strictly procedural, with testimony and presentation of evidence expected to begin in August or September, it will mark the first joint appearance of the defendants in court, 32 years after the Khmer Rouge were kicked out of power in 1979 with the help of a Vietnamese invasion and for more than a decade waged a bloody insurgency against the Phnom Penh government.

Tribunal began in 2006

Pol Pot escaped justice with his death in 1998, then a prisoner of his own comrades as his once-mighty guerrilla movement, in jungle retreat, was collapsing.

The tribunal, officially known as the Extraordinary Chambers in the Courts of Cambodia, started operations in 2006. Its first defendant was Kaing Guek Eav, also known as Duch, commandant of Tuol Sleng prison, where only a handful of prisoners survived. Up to 16,000 people were tortured under Duch's command and later taken away to be killed.

Duch, now 68, was sentenced to 35 years in prison for war crimes and crimes against humanity. His sentence was reduced to a 19-year term because of time previously served and other technicalities, bringing angry criticism from victims who called the punishment too lenient. Cambodia has no death penalty.