A son of the governor of a southern Philippine province on Thursday surrendered himself to the authorities after he and his family clan are tagged as prime suspects in Monday's massacre of at least 57 unarmed civilians.

Andal Ampatuan Jr., son of Maguindanao Governor Andal Ampatuan Sr. and mayor of Unsay town, is now in custody after military took over the provincial capitol and two municipality halls in the province, government officials said.

Local television footage shows Ampatuan Jr. walking down an Air Force helicopter at the airport of General Santos city. He is greeted by Justice Secretary Agnes Devanadera.

In a press conference held in Manila, National Police Director General Jesus Verzosa said Ampatuan is a suspect in the massacre and the police are determined to conduct an impartial probe.

The massacre occurred as around 100 pro-government militiamen herded a caravan of about 60 journalists, lawyers, and supporters of Buluan vice town mayor Esmail Magundadatu to a remote area in broad daylight before attacking them with M-16 rifles and machetes.

Police and soldiers have discovered 57 bodies at the massacre site. Some victims had their hands tied at the back. And local Red Cross personnel said some women apparently had been raped before the execution.

The convoy is expected to witness Magundadatu's wife to register him as the governor candidate in next year's provincial election.

Police initial findings indicated that pro-government militiamen loyal to the Ampatuan family were involved in the killing.

"We don't want anybody to doubt our investigation. We are trying to determine the motive behind this barbaric act," Verzosa said.