Philippine troops took the patriarch of a powerful clan — a former governor — into custody Saturday after the president put his southern province under martial law to hunt down suspects in the country's worst political violence.

The head of the clan, Andal Ampatuan Sr. and at least six other family members who have ruled impoverished Maguindanao province unopposed for years are the main suspects in the Nov. 23 attack on a political rival's convoy in which 57 people, including 30 journalists, were killed.

Ampatuan's son, Andal Ampatuan Jr., turned himself in last week and was charged with multiple counts of murder.

President Gloria Macapagal Arroyo on Saturday put Maguindanao under martial law, allowing troops to make arrests without court warrants and restore order, her top Cabinet member, Executive Secretary Eduardo Ermita, told reporters.

Soldiers have already seized stockpiles of weapons and ammunition in raids on homes belonging to the Ampatuans.

Armed forces chief Gen. Victor Ibrado said Ampatuan Sr., a three-term provincial governor, and his son Zaldy, governor of the Autonomous Region in Muslim Mindanao, which includes Maguindanao, were taken into military custody early Saturday.

TV reports said the patriarch was later taken to a hospital in southern Davao city.

The family has denied any involvement in the killings.

For several days, hundreds of security forces have surrounded the sprawling Ampatuan compounds in the provincial capital of Shariff Aguak to prevent the clan from escaping. They dug out an entire arsenal including mortar shells, light machine guns, assault rifles and bazookas buried near the patriarch's mansion, said armed forces spokesman Lt. Col. Romeo Brawner.

Some of the weapons and ammunition were marked as belonging to the national police and the Department of National Defense. Three armored personnel carriers also were seized, Brawner said.

Arroyo ordered an investigation of police and soldiers who may have supplied the weapons and ammunition, national police chief Jesus Verzosa said.

Arroyo also ordered the creation of an independent commission to oversee the dismantling of clan-dominated private armies in the country and investigate the carnage, Interior Secretary Ronaldo Puno said.

The 57 people killed in the massacre included the wife, sisters and supporters of Esmael Mangudadatu, who had sent them to submit his candidacy papers for governor, challenging the Ampatuan clan's iron-fisted control of the province.

Thirty journalists and their staff — the highest number of reporters slain in a single attack anywhere in the world — were also killed in the attack. Eight of the journalists were buried in a common grave Friday after a funeral procession in which their coffins were carried on a flatbed truck bedecked with flowers and a banner reading "We want justice." Residents threw more flowers as the procession passed.

The Ampatuan clan, notorious for running a large private army purportedly for protection against Muslim separatist rebels, has been allied with President Arroyo, who received crucial votes from the region during the 2004 elections.

Arroyo has promised swift justice in the killings and her ruling party has since expelled the Ampatuans.