Undeterred by the deadly attack that killed 57 in a campaign convoy, Ismael Mangudadatu filed to run for governor of Maguindanao province Friday, heavily guarded by police and soldiers.

His candidacy poses an unprecedented challenge to the Ampatuan clan, which was implicated in Monday's massacre. "Only death can stop me from running," he said.

A day after burying his wife, sisters and many other relatives, Mangudadatu traveled past the spot where they were killed along with supporters and journalists who were stopped and gunned down on their way to file election papers on his behalf. Many of the dead were dumped in mass graves.

"This symbolizes our freedom. I hope this will be the start of our liberation," he said, holding up his certificate of candidacy in front of reporters and hundreds of cheering followers.

His 50-vehicle caravan was escorted by soldiers, a police commander, a senior army general and hundreds of supporters. Mangudadatu was not part of Monday's convoy because of threats on his life. He sent female relatives in the belief that women would be spared.

Prosecutors, delayed by a two-day holiday, said they will file multiple murder charges Tuesday against Andal Ampatuan Jr., heir of the clan that has ruled Maguindanao for years.

Several witnesses came forward, including one of the gunmen who claimed he saw Ampatuan order the killings and fire his weapon, said Justice Secretary Agnes Devanadera.

Ampatuan told reporters from his detention cell in Manila that at the time of the massacre he was at the municipal hall in Datu Unsay township, where he's the mayor. He denied any role.

He turned himself in Thursday under threat of military attack on his family's compound.

The clan's patriarch, Andal Ampatuan Sr., and six other family members, including the governor of the autonomous Muslim region, are considered suspects and are not allowed to leave the country, Devanadera said. They were implicated in witnesses' statements, she said, refusing to elaborate.

Police said six senior officers, including the provincial police chief and his deputy, 20 members of Ampatuan township's police station and nearly 400 militiamen were in custody. Not all were considered suspects.

Interior Secretary Ronaldo Punourity said he would seek the replacement of the entire provincial political structure.

Faced with domestic and international outrage over the killings — the bloodiest in recent history — President Gloria Macapagal Arroyo authorized a crackdown on the clan that helped her and her allies win the 2004 presidential and 2007 senatorial elections.

Arroyo's ruling party expelled Ampatuan from their ranks, along with his father and a brother, while the president repeatedly vowed justice for the victims.

Devanadera said more than 20 women were among the slain, and the pants of some female victims were unzipped and lowered. Police said, however, that autopsies were not finished and reports of rape were not substantiated.

At least 22 journalists working for newspapers, TV and radio stations in the southern Mindanao region were among the dead. It was the most reporters killed in a single attack anywhere in the world, according to media groups.

Troops and police poured into the rural, impoverished province. Checkpoints were set up along highways and tanks deployed in the provincial capital.

Many hope that the fallout from Monday's massacre will lead to the end of the Ampatuan's violent grip on Maguindanao's politics.

"Our people were like strangled with a rope for a long time," said Arnold Fernandez, administrator of Buluan town, which is led by a Mangudadatu. "We seem to be moving to a new era."

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Associated Press writers Oliver Teves, Teresa Cerojano and Hrvoje Hranjski in Manila contributed to this report.

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