A heavily guarded Ismael Mangudadatu filed to run for governor of the Philippines' Maguindanao province, a day after burying his wife, sisters and other relatives killed in an election convoy attack.

His candidacy poses an unprecedented challenge to the Ampatuan clan implicated in Monday's massacre in which 57 people were killed.

"Only death can stop me from running," a defiant Mangudadatu said.

Mangudadatu yesterday travelled past the spot where his relatives, 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 symbolises 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.

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

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

Several witnesses came forward, including one gunman 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 is the mayor. He denied any role in the massacre.