MANILA, Philippines (UPDATE) - A powerful Muslim politician went on trial Wednesday, accused of murdering 57 people in the worst political massacre in the Philippines, AFP reporters in the courtroom said.

The first witness, a man named Lakmudin Suliya, climbed onto the stand after lower court judge Jocelyn Reyes rejected a motion by lawyers for main defendant Andal Ampatuan Jr. to move back the start of the trial 10 days.

Ampatuan and more than 100 gunmen allegedly stopped a convoy belonging to a political rival in the southern Philippines in November last year, killed 57 people and pushed the bodies into mass graves the suspects had dug beforehand.

Manette Salaysay, a relative of one of the victims who attended the court session, hailed the start of the trial, which had been delayed for 5 months.

"It is difficult to fight these devils," she told Agence France-Presse, referring to Ampatuan and 16 police officers also facing trial, accused of taking part in the massacre.

"We want to see the light of justice," Salaysay added.

Ampatuan, wearing a yellow prison shirt and flanked by plainclothes police, sat impassively behind his lawyers as the witness was sworn in.

The trial, held at a special courtroom built inside a maximum-security police jail in southern Manila, is being held amid allegations of witness intimidation and fears the case could drag on for years.

It was to start last week but Reyes moved it back 7 days to give the Ampatuan lawyers more time to comment on previous court rulings related to the case.

Rights groups and the victims' relatives have accused the Ampatuans of applying delaying tactics while ordering their men to terrorize witnesses.

Elaine Pearson, deputy Asia chief of the New York-based monitor Human Rights Watch, said 5 people with knowledge of alleged abuses by the Ampatuans had been killed since the massacre.

Among those was Suwaib Upham, a member of the clan's militia force who claimed to have taken part in the actual shooting and could place Ampatuan Jnr at the crime scene, it said.

Upham was gunned down in June before he was scheduled to be placed under a government witness protection program.

"The government needs to provide effective witness protection if these trials are going to succeed in bringing justice," said the rights monitor's Asia researcher Jessica Evans.

"Having the Ampatuans in custody seems to have weakened, but by no means eliminated, the Ampatuans' grip on power in Maguindanao," she said.

At least 30 journalists were among those shot dead in the massacre in Maguindanao province in the single biggest attack on the working press in history, according to global press watchdogs.

Five other Ampatuans, including the patriarch Andal Ampatuan Snr, are among the 196 people facing charges related to the massacre, although more than 100 of the alleged gunmen remain at large.

Ampatuan Jnr, then a local mayor, allegedly led the massacre to stop the rival from running against him for the post of Maguindanao province governor in this year's national elections.