Khmer Rouge survivors reacted with dismay Tuesday as a top regime leader walked out of his genocide trial for a second day and a co-defendant sought acquittal under a 15-year-old amnesty.

The elderly suspects' defiant attitude underlined the challenges facing Cambodia's UN-backed war crimes court in a case long awaited by victims of the 1970s totalitarian movement, which wiped out nearly a quarter of the population.

"Brother Number Two" Nuon Chea, wearing a woolly hat and his trademark sunglasses, refused to stay for the second day of proceedings focussed on preliminary legal objections by his co-defendant Ieng Sary.

Nuon Chea said he would only return to "actively participate" when his own case was discussed, and was escorted out of court by security guards.

On Monday the 84-year-old had left the courtroom after only half an hour in protest at the handling of the investigation and legal proceedings.

The four accused face charges including genocide, crimes against humanity and war crimes over the deaths of up to two million people from starvation, overwork, torture or execution during the Khmer Rouge's brutal 1975-79 rule.

"Nuon Chea is a bad person. I am quite disappointed with his behaviour," said farmer Thein Ouen, one of hundreds of people watching the hearing from the public gallery.

"I think he does not want to take part in the trial. We want him to tell us the truth about the Khmer Rouge, but he is trying to hide it."

The four elderly defendants, who also include former head of state Khieu Samphan and one-time social affairs minister Ieng Thirith, are allowed to be absent if they refuse to cooperate.

Ieng Sary, a former foreign minister, further angered victims of the regime with his claim that he should not be tried because he was granted a royal pardon and amnesty in 1996 exchange for leading a mass Khmer Rouge defection.

The complex trial, expected to take years, is seen as vital to healing the traumatised nation's deep scars.

But Va Chhorn, who was also watching from the public gallery, said of the defendants: "They are trying to avoid their responsibilities. This is not good."

"They committed mass killings," the 70-year-old added. "They should cooperate with the court to find justice for the people."

Ieng Sary, 85, was sentenced to death in absentia for genocide in a 1979 show trial conducted by the government installed after Vietnam invaded and occupied the country, ending the Khmer Rouge's bloody reign.

But the international face of the regime received a royal pardon and amnesty in 1996 upon surrendering to the government.

The prosecutors argued that Ieng Sary's pardon only saved him from his 1979 death sentence and the amnesty did not bar him from further prosecution, citing examples from other war crimes courts around the world.

The trial is the culmination of years of preparation by the tribunal, which was established in 2006 after nearly a decade of negotiations between Cambodia and the United Nations.

In its historic first trial, the tribunal sentenced former prison chief Kaing Guek Eav -- also known as Duch -- to 30 years in jail last July for overseeing the deaths of 15,000 people.

Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge emptied Cambodia's cities and abolished money and schools in a bid to create an agrarian utopia before they were ousted from the capital by Vietnamese forces.

The initial hearing will continue until Thursday, with more preliminary legal objections and talk of reparations for the nearly 4,000 victims taking part in the proceedings as civil parties.

Full testimony from the suspects, held at a purpose-built detention centre since their 2007 arrests, will not take place until late August at the earliest.