MANILA, Philippines (2nd UPDATE) - A Muslim clan blamed for a horrific political massacre in the Philippines was accused on Wednesday of killing more than 250 people during an eight-year reign of terror in Maguindanao province.

Explosive allegations of chainsaw murders and victims being buried alive were levelled by the nation's human rights chief as judicial authorities charged the leaders of the Ampatuan clan with rebellion.

Police also said they suspected at least 161 people, including rogue soldiers and policemen loyal to the Ampatuans, of direct involvement in last month's political massacre in Maguindanao that left 57 civilians dead.

Human rights commissioner Leila de Lima said the Ampatuans had murdered more than 200 other people during a brutal eight-year rule that President Gloria Arroyo's government had ignored because the clan were important allies.

"Everybody was aware (of the killings), but they have been tolerated," Commission on Human Rights chairwoman Leila de Lima told reporters.

"That is why we want full accountability now. The years of tolerance and neglect are over."

The patriarch of the clan, Andal Ampatuan Snr, had been governor of Maguindanao and an Arroyo ally since 2001.

Ampatuan Snr and and his relatives were allowed to run private armies as part of a government strategy to contain a long-running Muslim separatist insurgency in Maguindanao and other parts of the southern Philippines.

The Ampatuans were also a rich source of votes for Arroyo's ruling coalition.

De Lima said witnesses, who only dared to come forward after the November 23 massacre triggered the clan's downfall, had given horrifying accounts of people being buried alive and slain with chainsaws in "killing fields".

Arroyo ended the alliance only after the Ampatuan clan was accused of organising last month's massacre, which was allegedly carried out to stop a rival politician from running for provincial governor in next year's elections.

Police allege Ampatuan Snr's son and namesake, a local mayor, led a militia of more than 100 gunmen in abducting then shooting dead a convoy carrying female relatives of his political rival, plus a group of journalists.

Andal Ampatuan Jnr was detained three days after killings and has been charged with 25 counts of murder so far.

After turning against her former allies, Arroyo declared martial law in Maguindanao on Friday night and accused the Ampatuans of rebellion.

Amatuan Snr and four of his sons, who were among 62 people swept up in martial law raids over the weekend, were charged in a southern Philippine court on Wednesday with rebellion and could face life in prison.

The government says 3,000 of their former militia remain on the run and capable of further atrocities.

De Lima hit out at the government's long alliance with the Ampatuans, saying reports of political opponents and others going missing in Maguindanao had circulated for years.

"It was too dangerous for our investigators to validate these reports earlier," she said. "But now we are taking advantage of the military security to go and investigate."

De Lima said independent forensics experts deputised by her agency would begin excavating two alleged "killing fields" in the next two weeks.

"We are looking at a minimum of 200 (bodies)," de Lima said. "These are victims of the same clan and the private armies."

Among those missing and presumed killed was a former Ampatuan lawyer who had wanted to expose the clan's atrocities, she said. Others were regular peasants who fell out of favour with the family.

"Some were allegedly killed with the use of chainsaws, and others were buried alive," de Lima said, citing witnesses now in her agency's protection.

Five police officers who had direct knowledge of some of the murders were among those now willing to speak out against the Ampatuans, according to de Lima.

One of de Lima's aides said the rights commission could not divulge any more details about the 200-plus murders because it wanted to protect witnesses.