The killings appeared to be linked to an electoral challenge to the Ampatuan clan by the Mangudadatu family. Esmael Mangudadatu, the deputy mayor of the small town of Buluan, is planning to run for governor of Maguindanao next year, a position that Gov. Andal Ampatuan Sr. is believed to have promised his son. The victims were abducted while on their way to file paperwork to register Mr. Mangudadatu as a candidate.

The bodies recovered so far included members of the Mangudadatu clan, which controls a province just to the south of Maguindanao; at least 18 journalists; and several women who went along in the belief that the militias, following custom, would not hurt them.

Image Police officers stood guard on Wednesday as the bodies of massacre victims were dug up in Maguindanao Province. Credit... Erik de Castro/Reuters

As the scope of the massacre became clear, the Arroyo administration promised a swift investigation. The president “is enraged by these barbaric acts,” said a spokesman, Cerge M. Remonde. “She has literally thrown the full force of the law and has mobilized the security and police forces of the state to go after the perpetrators.” Mrs. Arroyo also declared a state of emergency in Maguindanao and an adjoining province this week.

Mrs. Arroyo came under intense pressure to go after the Ampatuans, with her opponents accusing her of treating her allies with too much leniency. Her political party expelled Mr. Ampatuan, his father and his brother Zaldy, the governor of an autonomous Muslim region, on Wednesday.

“This is unprecedented and historic,” said Kim Bagundang, a Maguindanao resident and the president of the nonprofit Liguasan Youth Association for Sustainable Development, noting that the people of Shariff Aguak had never witnessed a member of the clan seemingly vanquished. “This province and this town”  which is run by a cousin of Mr. Ampatuan  “is their kingdom. They rule absolutely.”

Critics have attributed the massacre to the power of the Ampatuan dynasty in this troubled province. Until this week, Andal Ampatuan Sr., the governor, maintained a private militia of more than 200 men, military officials have said. He treated the province as his own domain, they say, carving out new towns and letting his sons run them. One such town is Datu Unsay, which he created. His son, who is known commonly as Datu Unsay, became mayor, and the town was named for him.

The army announced on Wednesday that it would disband the 200-member militia, and it deployed 500 extra troops from the central Philippines to Maguindanao, which is part of the long-troubled region of Mindanao.