The long-awaited trial of top Khmer Rouge leaders accused of genocide and other atrocities finally gets under way in Cambodia on Monday, but one of the four accused will be notably absent.

More than three decades after the country's 'Killing Fields' era, a UN-backed war crimes court is set to hear opening statements in the case against 'Brother Number Two' Nuon Chea, ex-head of state Khieu Samphan and former foreign minister Leng Sary.

Led by 'Brother Number One' Pol Pot, who died in 1998, the communist Khmer Rouge wiped out nearly a quarter of the Cambodian population through starvation, overwork and executions in a bid to create an agrarian utopia.

Court monitor Clair Duffy from the US-based Open Society Justice Initiative said Monday's opening would be a landmark moment for Cambodia.

"It's a really important overview of what these people are alleged to have done," she said.

Missing from the courtroom will be Ieng Thirith -- the regime's 'First Lady' and the only female leader to be charged by the court -- after she was ruled unfit for trial on Thursday because she has dementia.

Despite this last-minute drama, next week's proceedings are considered a major event in the still-traumatised nation's quest for justice.

Hundreds of Cambodians will travel to see the defendants in the dock, and events will be broadcast on television.

"We really want justice to be served," said prominent Khmer Rouge survivor Chum Mey, 80. "The victims want the accused to tell the truth for the sake of history."

It remains to be seen how much satisfaction victims will get from the legal process, with all three denying charges of war crimes, crimes against humanity and genocide over the deaths of up to two million people during the regime's 1975-79 reign of terror.

Amid fears that not all of the accused, who are in their 80s and suffer from varying ailments, will live to see a verdict, the court recently split their complex case into a series of smaller trials.

But during the four days of opening statements the prosecution and the defence will address all the accusations against them.

"We will be setting out the roadmap for these cases so it's a very important day. It's a summary of the evidence we anticipate the court will hear," said international co-prosecutor Andrew Cayley, who will be first to speak on Monday.

Of the accused, only Khieu Samphan has indicated he will cooperate, telling the court in June that while he was not 'fully knowledgeable' about everything that happened, he would help to find 'the truth'.

Nuon Chea walked out of the June hearing after telling the court he was 'not happy'. His lawyers say he will make opening remarks on Wednesday, but it is not known whether he will answer questions.

And Ieng Sary, frequently the only point of contact between the secretive communist regime and the outside world, told the court last month he does not intend to testify.

Khmer Rouge survivor Theary Seng told AFP she was 'frustrated beyond words' that only Khieu Samphan looked likely to shed light on what happened.

"The people want to know who is behind the Khmer Rouge, we want to see and understand the larger picture and we're not going to get that," she said.

But international co-prosecutor Andrew Cayley, who will be first to speak on Monday, said defendants had the right not to say anything during their trial.

"The case can be proven against these accused without them giving evidence," he said.

The first mini-trial focuses on the forced movement of population and related charges of crimes against humanity, and will begin hearing evidence on December 5.

The case against the three is the court's second and most important after it sentenced former Khmer Rouge prison chief Duch to 30 years in jail last year for overseeing the deaths of some 15,000 people.

It comes at a critical time for the perennially cash-strapped tribunal, which has been mired in controversy over its handling of two possible new cases against five lower-ranking cadres that are strongly opposed by the government.

The issue came to a head in October with the shock resignation of a German investigating judge, one of two responsible for issuing indictments, citing government interference.

The move prompted a visit to Cambodia by the UN's top legal chief, who urged Phnom Penh to refrain 'from interfering in any way whatsoever with the judicial process'.

The government has denied meddling but Prime Minister Hun Sen -- himself a former cadre -- has made it clear he wants the court's work to end with case two, even saying last year that more trials were 'not allowed'.