Andal Ampatuan Jr., centre, a town mayor suspected in Monday's massacre in the southern Philippines, is escorted by police in General Santos City after surrendering. (Associated Press)

Philippine authorities have laid murder charges against the member of a powerful family suspected in Monday's massacre of 57 people travelling in the election caravan of a political rival, according to reports.

Andal Ampatuan Jr., a town mayor in the province of Maguindanao on the country's southern island of Mindanao, turned himself in on Thursday to presidential adviser Jesus Dureza in the provincial capital, Shariff Aguak.

Interior Secretary Ronaldo Puno said he had warned Ampatuan's family they risked a military attack unless they turned him over by midday Thursday.

The arrest came a day after President Gloria Macapagal Arroyo's ruling Lakas Kampi CMD party, facing local and international pressure to crack down on regional warlords, expelled Ampatuan and two other members of his family.

The Ampatuan family holds a number of regional government positions and has been close allies of the president, delivering crucial votes to Arroyo and her circle in the 2004 presidential and 2007 senatorial elections. Also expelled from the party were Ampatuan's father, Datu Andal Ampatuan Sr., who is the governor of Maguindanao, and brother Zaldy, the governor of the Autonomous Region in Muslim Mindanao.

The southern Philippines has a history of election-related violence due to the presence of armed groups. Muslim insurgents have been waging a campaign since the 1970s, while some politicians maintain private militias. But the scale of atrocity in Monday's massacre was shocking, even by the standards of the often lawless province of Maguindanao.

Ampatuan denies involvement

The dead from Monday's massacre include the wife, family and supporters of Ismael Mangudadatu, vice-mayor of Buluan township, as well as 18 journalists following the caravan. The vehicles were on their way to register Mangudadatu as a candidate for provincial governor in Maguindanao, for which he would be competing against Ampatuan Sr.

The caravan was stopped by armed gunmen, according to witnesses. Soldiers and police later found the victims' bodies sprawled on the ground or shot in vehicles at the scene, and later uncovered dozens more in a mass grave.

Mangudadatu, who was not in the caravan, said four witnesses told him the gunmen were followers of Ampatuan Jr.

Asked by reporters Thursday whether he was involved in the killings, the younger Ampatuan replied: "There is no truth to that. The reason I came out is to prove that I am not hiding and that I am not guilty."

As a helicopter carrying Ampatuan took off from Maguindanao's provincial capital for General Santos City to the south, shots rang out but the aircraft was not hit, according to military commander Lt.-Gen. Raymundo Ferrer. It wasn't clear who fired the shots.

National police director Jesus Verzosa said six senior officers, including the provincial police chief and his deputy, as well as 20 members of the local police station and 347 militiamen were in custody as part of the investigation into Monday's massacre, but that not all were considered suspects.

With files from The Associated Press