Two Khmer Rouge leaders on trial for genocide have told the UN-backed court it has no authority to prosecute them.
The regime's head of state Khieu Samphan said the prosecution case was "monumentally biased" and relied on accounts from newspapers.
Ex-Foreign Minister Ieng Sary reiterated that he had received a royal pardon so should not be on trial.
They are accused of leading a campaign of mass murder in Cambodia from 1975-1979 in which up to 2.2 million died.
The Khmer Rouge regime tried to establish a Maoist utopia by forcing everyone to live as peasants, and exterminating everyone they regarded as enemies.
The policies plunged the country into a humanitarian catastrophe - the regime was eventually swept from power by invading Vietnamese forces.
The long-awaited trial of three defendants began on Monday.
The third defendant is Nuon Chea, known as Brother Number Two, who was the right-hand man to supreme leader Pol Pot.
All three, who are in their 80s, deny the charges they face.
Khieu Samphan dismissed the evidence against him as "fairytales" and launched a lengthy attack on the trial process and the prosecutor.
He said the prosecution "wants my head on the block".
He told the court that the Khmer Rouge had launched a successful resistance movement against an unpopular government and US forces, and was backed by the Cambodian people.
He also said he was not responsible for every decision made by the regime, and could not have known about every death.
Nuon Chea made similar arguments in his statement to the court in Tuesday's session.
Ieng Sary read out a short statement saying he would reject the court's authority until Cambodia's domestic courts had ruled on the validity of his 1996 royal pardon.
Former King Norodom Sihanouk granted the pardon as part of a deal which resulted in the final surrender of the Khmer Rouge.
However, all three defendants have said they will continue to participate in the proceedings.
This case against these three men has been broken up into several mini-trials, with the first hearing set to judge on the offence of enforced removal of people from the cities.
The defendants are all in their eighties - concern that they might die has forced the tribunal to split the cases in the hope of gaining at least one conviction.
The trial is only the second to be held at the genocide court. Last year former Khmer Rouge prison chief Duch - who oversaw the notorious Tuol Sleng jail - was jailed for 19 years.