MANILA: President Gloria Arroyo on Saturday lifted martial lawin a southern Philippine province where an election-linked massacre left 57 people dead, senior aides said, with over 500 suspects now arrested.

The regime imposed eight days ago in Maguindanao province ends at 9:00pm (1300 GMT), the government announced after Arroyo met her top security advisers in the capital Manila.

Martial rule was imposed in the country's second-poorest province to allow arrests without court orders, helping police detain members of a powerful Muslim clan led by Andal Ampatuan Snr, Arroyo's former political ally.

His son Andal Ampatuan Jnr had earlier been arrested and charged with multiple counts of murder for allegedly ordering the November 23 kidnapping and shooting of 57 people, including journalists and relatives of a local rival.

The rival, Esmael Mangudadatu, alleged the killings were carried out to prevent him running against the younger Ampatuan for governor of Maguindanao next year.

The Ampatuans were later expelled from Arroyo's ruling Lakas Kampi CMD coalition and the clan patriarch was among 24 people arrested and separately charged with rebellion after martial law took effect.

The government also seized a vast array of weaponry from the Ampatuans' large group of bodyguards.

"In view of these accomplishments... the cabinet group recommended the lifting of Proclamation 1959 and the president has approved the recommendation," Executive Secretary Eduardo Ermita told a news conference.

Ermita said 529 people are in custody after eight days of military and police operations in the province, most of them members of the Ampatuan clan or their armed militia.

He said 247 other suspects were being investigated for allegedly taking part in the massacre and could face murder charges, while up to 638 people could be charged with rebellion.

Manila acknowledged it had allowed the clan to build its own army to help overstretched government forces in Mindanao fight communist and Islamist militants with alleged Al-Qaeda ties.

Ermita said government forces had "cleared" seven Maguindanao towns of armed Ampatuan militia, with local government units functioning again after the previous officials, many Ampatuan clan members, were replaced.

A military force numbering 4,000 troops will remain in Maguindanao while security checkpoints will remain in place on its roads, said the martial law administrator, Lieutenant-General Raymundo Ferrer.

"We shall continue with the military and police operations against the remaining suspects and strengthen even more our checkpoints to ensure that the people are insulated against hostile acts of the private armed groups that we intend to disarm and dismantle," he told reporters.

However, police will now need court orders to arrest suspects or search private property for weapons or other evidence in the massacre and rebellion cases.

Arroyo's lifting of martial law came amid allegations she was using the measure as a cover to allow her to stay in power beyond the May 2010 presidential election.

Senate President Juan Ponce Enrile welcomed the decision.

"All of these are in the exercise of her powers. If she can suspend the writ of habeas corpus (protection against illegal detention) she can also proclaim martial law. She can also end it," Enrile told DZMM radio.

The Ampatuans' local rival Mangudadatu, who lost his wife in the massacre, told the station he was satisfied with the murder investigation, but insisted not all those responsible for the slaughter were in custody.

"There are more or less 20 others still at large," he said without elaborating.