The Philippines on Saturday announced the imposition of martial law in a southern province to quell a rebellion by a powerful 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 said. "It was practically an overthrow of government."

Arroyo's controversial move is the first time martial law has been declared in the Philippines since the reign of dictator Ferdinand Marcos, who had the whole of the country under martial law from 1972 to 1981.

Authorities insisted martial law was necessary to rein in swarms of heavily armed gunmen loyal to the Ampatuan clan who had threatened violence if their leaders were taken into custody. Related article: Farmers flee over fears of violence

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

More than 4,000 government soldiers were deployed in Shariff Aguak, the provincial capital, and other Ampatuan strongholds, the military said.

By nightfall on Saturday, 32 people had been taken into custody, including five members of the Ampatuan family and 20 of their militiamen, national police chief Director General Jesus Verzosa told reporters in Manila.

The militiamen were arrested in a raid on an Ampatuan warehouse that also netted 340,000 rounds of ammunition for M16 assault rifles, Verzosa said.

But he warned that the many gunmen not yet rounded up were likely to fight back.

"So far, there has been no firefight but we expect there will be an engagement because they are armed," he said.

Andal Ampatuan Jnr, a son of the 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 farming area near Shariff Aguak town.

Police allege he and 100 of his men shot dead the occupants of a convoy that included relatives of his rival for the post of 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 running for office.

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

The cache 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.

Devanadera said local governments had stopped functioning as a result of efforts by forces loyal to the Ampatuans to foment rebellion.

But Arroyo's critics said martial law was not justified and might be unconstitutional.

They also warned it might be a prelude to her seizing similar control of other parts of the country or even part of a bid to remain in power after the constitution requires her to step down next June.

"We believe there's no basis for the implementation of martial law," said opposition Senator Benigno Aquino, the front-runner in 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 controversial containment strategy against the insurgents.

Andal Ampatuan Snr and Jnr were members of Arroyo's ruling coalition until last week.