The former president and vice-president of Indonesia's national airline went on trial today accused of conspiracy to murder one of their passengers, a human rights activist.

Munir Said Thalib, a critic of Indonesia's powerful military, died of arsenic poisoning while on board a Garuda flight in September 2004.

Indra Setiawan, the former head of Garuda, and his deputy, Rohainil Aini, are being tried in separate courtrooms at Central Jakarta District Court on charges of helping the main suspect of the killing, an off-duty Garuda pilot. They face up to 20 years in jail or a possible death sentence if convicted.

The trials are being watched closely as a test of the independence of Indonesia's legal system, as Munir had been outspokenly critical of the way the regime suppressed rebel movements in dissident provinces such as Aceh and Papua.

Pollycarpus Priyanto, the off-duty Garuda pilot who was aboard the same flight, was convicted in 2005 of carrying out the murder and sentenced to 14 years.

The Supreme Court cleared him on appeal last year, however, ruling that there was a lack of evidence and witnesses to support the conviction.

Prosecutors are now attempting to overturn that decision, claiming that they have uncovered new evidence suggesting that Mr Priyanto poisoned Munir while in transit at Changi airport in Singapore.

At an earlier hearing, Mr Setiawan said that he received a letter signed by the deputy head of the Indonesian spy agency before Munir's death, asking him to allow Mr Priyanto to be a flight security officer. Mr Setiawan said that he had lost the letter.

Prosecutors say they will produce evidence that Ms Aini signed a letter assigning Mr Priyanto as a security officer on Munir's flight.

Collectively, they will argue, Mr Setiawan and Ms Aini forged documents enabling Mr Priyanto to gain access to his victim and give him a drink laced with arsenic.

Mr Setiawan has argued that he should not be tried for conspiracy to murder when the main suspect for the murder has been acquitted.

"The prosecution charges are unclear and should be dismissed," said Hari Wicaksono, Mr Setiawan's defence lawyer.

Human rights activists fear that Munir was killed with the collusion of the State Intelligence Agency, because of his work in exposing abuses by the Indonesian security forces.

Under the 32-year dictatorship of President Suharto, which ended in 1998, state-sponsored killings were common, and security officers were above the law.

Munir's wife toured the world after the killing, meeting rights activists and politicians, and extracted a promise from President Susilo Bambang Yudhoyono that his killers would be punished.

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