A UN-backed Cambodian war crimes court has opened the trial of four top leaders of the Khmer Rouge who are accused of genocide and other atrocities in the late 1970s.

The defendants, all now elderly, were among the inner circle of the late Pol Pot.

'Brother Number One', Pol Pot, was the French-educated architect of the Khmer Rouge's ultra-Maoist 'Killing Fields' revolution that killed an estimated 1.7m Cambodians from 1975-1979.

'Brother Number Two' Nuon Chea, former President Khieu Samphan, ex-Foreign Minister Ieng Sary and Ieng Thirith, a former social affairs minister, showed no emotion as opening statements to the UN-backed tribunal were read before a packed auditorium in proceedings screened on national television.

The four are charged with committing crimes against humanity and genocide.

They are accused of a litany of crimes under both international and Cambodian laws, including murder, enslavement, religious and political persecution, inhumane treatment and unlawful imprisonment.

All four defendants were expected to enter not guilty pleas.

The opening session was dominated by moves from Ieng Sary's lawyers to have his case thrown out for double jeopardy.

He was sentenced to death by a court created by Vietnamese invaders in 1979 and pardoned by Cambodia's then King Norodom Sihanouk 17 years later.

'An individual cannot be tried twice for a crime that a court already acquitted or convicted him for,' lawyer Ang Udom said. 'Bringing Ieng Sary to trial again is a violation.'

The pardon for Ieng Sary, a reclusive guerrilla leader, came as part of a peace deal between warring factions in Cambodia.

Prosecutors are expected to argue the pardon was for the death sentence, not the charges he currently faces.

Up to 2m people died due to starvation, overwork, torture or execution during the Khmer Rouge's four years of rule in the country.

The communist group had sought to create an agrarian, self-sufficient society during its reign and abolished religion, schools and currency as part of this drive.

The Khmer Rouge was eventually driven from power in 1979 by Vietnamese troops and former regime members who defected, including Hun Sen, now Cambodia's prime minister.

Sen was a mid-level military commander until fleeing to Vietnam in 1977. Under him, the Cambodian government fought the Khmer Rouge until the movement collapsed in the late 1990s.