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An army trooper stays alert at a military checkpoint Wednesday in Ampatuan, Maguindanao province, in the southern Philippines. ((Aaron Favila/Associated Press))

Authorities say a scion of a powerful clan suspected in the massacre of 57 people in an election caravan in the southern Philippines has turned himself in.

The dead in Monday's massacre include dozens of journalists and the wife, family and supporters of a candidate for governor who wanted to challenge the rival Ampatuan clan, which has ruled the lawless province of Maguindanao unopposed for years.

Army Lt. Gen. Raymundo Ferrer says Andal Ampatuan Jr., a town mayor, surrendered himself to presidential peace adviser Jesus Dureza in the provincial capital.

Ampatuan's family is denying allegations of his involvement in the slayings.

Philippine President Gloria Macapagal Arroyo declared a national day of mourning Wednesday and promised justice for the victims.

"This is a supreme act of inhumanity that is a blight on our nation," she said in a statement. "The perpetrators will not escape justice. The law will haunt them until they are caught."

The victims were travelling in a convoy of vehicles to file papers nominating Ismael Mangudadatu for next year's election for governor of the southern province of Maguindanao.

Witnesses claim that dozens of gunmen aligned to Ampatuan Jr. stopped the convoy and killed its occupants.

Regional police commander Josefino Cataluna said three vehicles were found buried in one of the mass graves. Investigators said the vehicles had been crushed by a backhoe and then buried.

Police said they are looking into reports that Ampatuan carried out the attack with the help of four police commanders, plus dozens of police and pro-government militiamen.

A national police chief said the four commanders had been relieved of duty and confined to their camp during the investigation.

Human Rights Watch is worried that the Philippine government's relationship with the clan could hinder an impartial investigation.

With files from The Associated Press