Local politician and mayor Andal Ampatuan Junior has been formally charged with mass murder after authorities accused him of ordering gunmen, soldiers and police to kill at least 57 defenceless people in an organised politically-motivated massacre last Monday.

Justice Secretary Agnes Devanadera also announced the suspect's father, the leader of the Ampatuan clan and governor of Maguindanao province, was among eight other members of the powerful family under investigation and not allowed to leave the country.

On Monday, a convoy of aides and relatives to Esmael Mangudadatu - a rival politician who is contesting the polls in May 2010 - was on its way to an election office to file Mr Mangudadatu's nomination papers. His wife's caravan, which included dozens of journalists, was stopped by some 100 armed men. Fifty seven were shot and hacked to death. The victims included 27 journalists and several people who were not part of the convoy - but apparently killed to eliminate witnesses. The bodies were recovered from shallow graves in nearby fields.

Mr Ampatuan Junior has protested his innocence, blaming the separatist Moro Islamic Liberation Front but eyewitnesses say he was personally involved. According to some reports, he actually fired at Mr Mangudadatu's supporters himself. Mr Ampatuan Junior surrendered to authorities on Thursday and is being held in custody in Manila.

Mr Mangudadatu said Monday's massacre had not altered his plans to run for the post of governor of Maguindanao province and registered his candidacy earlier today.

Maguindanao is part of Mindanao island, where Muslim clans rule vast areas backed by their own private armies, often out of the national government's control. The Ampatuan clan has a longstanding feud with the Mangudadatu family. Two army chiefs directly responsible for the area where the massacre occurred have been removed from their posts and placed under investigation.