The son of a Philippine clan boss suspected of the election-related massacre of 57 people has turned himself in to authorities on Thursday, international media reported.

Andal Ampatuan, Jr., the son of a key local supporter of the incumbent Philippine president, Gloria Arroyo, is accused of masterminding Monday's massacre on the Philippines' southern island of Mindanao, the Times online edition said.

Ampatuan, who is the mayor of a town in the Maguindanao province, turned himself into authorities after police put him on a wanted list in suspicion of the crime. He has dismissed the accusation, and the paper quoted him as saying "there is no truth to that."

"The family voluntarily surrendered him and they agreed that he will be questioned," military commander Lt. Gen. Raymundo Ferrer was quoted by BBC as saying.

Besides Ampatuan, Jr., 20 other suspects have been reportedly arrested over the massacre.

According to the Armed Forces of the Philippines, about 100 gunmen from the so-called private army of Andal Ampatuan, the Maguindanao province's incumbent governor, intercepted on Monday morning a convoy of political rivals in the forthcoming provincial elections near the town of Shariff Aguak.

The convoy was on its way to file the nomination of Esmael "Toto" Mangudadatu to contest the governorship of Ampatuan in the Maguindanao province next year.

A number of females were raped before being shot and there have also been reports of beheadings, national media said. The candidate's wife was also among the dead. Media reports also said 12 journalists were among those slaughtered.

On Wednesday, Philippine President Arroyo declared a national day of mourning for victims of the massacre.

A state of emergency was earlier announced in several southern Philippine cities that allowed police to make wider arrests, impose a curfew and set up police checkpoints.

According to the Philippine Daily Inquirer, 121 people were killed in the country ahead of the 2007 parliamentary elections, and the 2004 election campaign was accompanied by 148 murders.

MOSCOW, November 26 (RIA Novosti)