BULUAN, Maguindanao—Philippine security forces on Thursday poured into the territory of a powerful clan accused of a massacre that left 57 people dead, arresting some of its gunmen and disarming others.

Authorities said they expected the alleged mastermind of Monday's slaughter to turn himself in by the end of the day, following intense pressure for President Gloria Macapagal-Arroyo's government to take decisive action against the clan.

However, indicating the situation in Maguindanao province remained extremely volatile, the military said most of the Ampatuan family's militiamen alleged to have carried out the massacre had no intention of giving themselves up.

"Most of the armed group that perpetrated this crime have run away towards the mountainous area of Maguindanao," military spokesman Lieutenant Colonel Romeo Brawner said on ABS-CBN television.

"That is where we are conducting our pursuit operations."

An AFP photographer witnessed armored personnel carriers patrolling highways in the southern province of Maguindanao, and television footage showed police commandos surrounding buildings in major towns controlled by the clan.

Police announced the operation had secured the first arrests since the brutal explosion of political violence, saying several gunmen were detained on Thursday morning.

"We don't have an exact number (of those arrested) but our policemen in the area have arrested several," national police Director General Jesus Verzosa said on DZBB radio.

He identified the suspects as militiamen under the control of Andal Ampatuan Jr., a mayor of a town in Maguindanao who is accused of masterminding Monday's killings.

"Andal Ampatuan Jr. is a suspect. He has sent feelers and Secretary Dureza will accompany him to submit to an investigation," Verzosa said, referring to President Gloria Macapagal-Arroyo's adviser on the southern island of Mindanao.

Ampatuan Jr. is the son of Maguindanao's governor, a Muslim clan chief who commands his own private army and until this week was a close ally of Arroyo's ruling coalition.

Maguindanao is a part of the lawless Mindanao island, where Muslim clans rule vast areas backed by their own private armies, often out of the national government's control.

Monday's massacre occurred after about 100 Ampatuan gunmen allegedly abducted a convoy of aides and relatives of a rival politician, Esmael Mangudadatu, plus a group of journalists.

The victims were snatched as they were traveling in a six-vehicle convoy to nominate Mangudadatu as the opposition candidate for provincial governor in next year's elections.

They were shot at close range, some with their hands tied behind their backs, and dumped or buried in shallow graves on a remote farming road close to a town bearing the Ampatuan name.

Fifty-seven bodies have been recovered so far, and police were expected to continue searching Thursday for more potential victims.

Ampatuan Sr. had been grooming his son to take over as governor of Maguindanao.

The victims' relatives alleged the Ampatuans organized the murders so that Mangudadatu would not run for that post.

The ruling Lakas Kampi CMD coalition late on Wednesday expelled both Ampatuans from the party.

Ampatuan's brother, Zaldy, who was governor of an autonomous region on Mindanao that included Maguindanao, was also expelled.

"(They were) expelled for their failure to uphold party ideals and principles in their area of jurisdiction," the coalition's nomination for president in next year's elections, Gilberto Teodoro, said in a statement.