The Philippines said today it had imposed martial law in a southern province to quell a rebellion by the region's ruling clan accused of being behind the massacre of 57 people.

President Gloria Arroyo placed Maguindanao province under military control late on Friday in an effort to contain heavily armed militias belonging to the provincial governor and other members of his Muslim clan, authorities said.

"There's a rebellion in the area," Justice Secretary Agnes Devanadera told a nationally televised news conference as she outlined the reasons for the imposition of martial law.

"It was practically an overthrow of government."

Martial law was also implemented to make it easier to bring members of the powerful Ampatuan clan into custody, presidential spokesman Cerge Remonde told reporters.

"President Gloria Macapagal Arroyo has taken this bold step in answer to the cry for justice of the (relatives of the) victims of the now infamous Maguindanao massacre," Remonde said.

It was the first time martial law had been declared anywhere in the Philippines since the reign of dictator Ferdinand Marcos. He had the whole of the country under martial law from 1972 to 1981.

Within hours of martial law being declared, special forces detained the province's governor and patriarch of the clan, Andal Ampatuan Snr, who since 2001 had ruled Maguindanao with the backing of a private army.

More than 4,000 soldiers were deployed across Maguindanao, and an AFP photographer on the outskirts Shariff Aguak, the provincial capital, witnessed army checkpoints checking all vehicles travelling on the roads.

One of the clan chief's sons, Zaldy Ampatuan, the governor of an autonomous Muslim area in the southern Philippines, was also taken into custody on Saturday morning, along with two other relatives, according to the military.

Andal Ampatuan Jnr, another son of the clan patriarch, is already in a Manila detention centre after being charged with 25 counts of murder for the November 23 massacre that took place in a remote area of Maguindanao.

Police allege Ampatuan Jnr and 100 of his gunmen shot dead the occupants of a convoy that included relatives of his rival for the post of Maguindanao governor in next year's elections, as well as a group of journalists.

The rival, Esmael Mangudadatu, said the killings were carried out to stop him from running for office.

The armed forces chief of staff, General Victor Ibrado, and other military chiefs said martial law was imposed because members of the Ampatuans' private armies had threatened to attack civilians and government installations.

"By their sheer number they are really a threat to the peace and order in the province," Ibrado said.

Weapons enough to equip 1,000 soldiers

The military said one of the triggers for martial law was the discovery on Thursday of a huge cache of weapons buried just a few hundred metres (yards) from the Ampatuans' family compound in Shariff Aguak.

The weapons inventory included three anti-tank recoilless rifles, mortars, machine guns, rifles and pistols, and thousands of rounds of ammunition - enough to arm two battalions or about 1,000 soldiers.

Presidential spokesman Remonde insisted that martial law would be lifted as soon as possible, with a maximum limit of 60 days.

But Arroyo critics immediately hit out at the move, warning it may be a prelude to her seizing similar control of other parts of the country or even a bid to remain in power after the constitution requires her to step down.

"We believe there's no basis for the implementation of martial law," said opposition Senator Benigno Aquino, who is the front-runner to win next year's presidential elections.

Muslim rebels fighting for an independent homeland have been waging a rebellion on Maguindanao and other parts of Mindanao island since the late 1970s. The conflict has claimed more than 150,000 lives, the military says.

Arroyo's government has used Muslim clans such as the Ampatuans to rule these areas, and allowed them to build up their own armies as part of a containment strategy against the insurgents.

However critics have said the strategy has allowed these regional rulers to become warlords who believe they can act with impunity.

- AFP