The Philippine president imposed martial law Saturday on a southern province and security forces detained the patriarch of a powerful clan and three of his sons, accusing them of massacring 57 people and fomenting a rebellion.

President Gloria Macapagal Arroyo was accused of overreacting by invoking emergency powers in response to the country's worst incident of political violence. It was the first time martial law has been declared in the country since the late dictator Ferdinand Marcos imposed it more than three decades ago.

The head of the clan, Andal Ampatuan Sr. — a former governor — and at least six other family members who have ruled impoverished Maguindanao province unopposed for years, are the main suspects in the Nov. 23 attack on a rival's convoy. Some 30 journalists were among the dead. The family has denied involvement.

The Ampatuans, notorious for running a large private army, have previously been allied with Arroyo, who received crucial votes from the volatile southern region during 2004 elections. Arroyo's ruling party expelled the clan after the massacre.

The martial law proclamation allows troops to make arrests without court warrants and to restore order, Arroyo's top Cabinet member, Executive Secretary Eduardo Ermita, announced on national television early Saturday.

The last Philippine leader to declare martial law was Marcos, whose nationwide declaration in 1972 paved the way for his one-man rule that ended with his ouster in 1986.

Under the post-Marcos constitution, Arroyo can enforce martial law for 60 days, unless Congress revokes or extends it.

Former President Fidel Ramos, who was a supporter of Arroyo but has recently been critical of her actions, described her move as "overkill."

In Saturday's announcement, Ermita cited military reports as saying heavily armed supporters of the Ampatuans had "plans to undertake hostile action" if clan members were arrested.

"We felt that this is a very imminent threat, so we recommended this proclamation," military chief of staff Gen. Victor Ibrado said. "By their sheer number, they are really a threat to the peace and order of the province."

Justice Secretary Agnes Devanadera said the arrested men would be charged with rebellion, which carries a penalty of up to 40 years in prison.

"We evaluated very closely the facts and circumstances and information that were reported to us and there is no mistake in the analysis — it was a looming, and in fact it was already, practically an overthrow of government," she said.

But CenterLaw, a group of human rights lawyers, said there were insufficient grounds for martial law and would challenge it in the Supreme Court.

"The Constitution limits the grounds to insurrection, rebellion and invasion. None of these grounds are existent," it said in a statement. "We call on the citizenry to be vigilant for the defense of their civil liberties."

For several days, hundreds of security forces have surrounded the sprawling Ampatuan compounds in Shariff Aguak, the provincial capital. Soldiers on Saturday seized a large cache of weapons and ammunition that was buried near one of the homes.

A warehouse owned by the patriarch yielded armored and military and police vehicles, M-16 assault rifles and tens of thousands of rounds of ammunition, said Col. Leo Ferrer, a local army commander.

Security forces had earlier found mortar shells, light machine guns, assault rifles and bazookas buried near the patriarch's mansion, some of it marked as belonging to the police or military.

Ibrado said Ampatuan Sr., a three-term provincial governor, and his son Zaldy — governor of the Autonomous Region in Muslim-majority Mindanao, which includes Maguindanao — were taken into military custody early Saturday. Two other sons and five other men sought in connection with the massacre were also detained, police said.

Another son, Andal Jr., turned himself in last week and was charged with multiple counts of murder.

The patriarch was taken later Saturday to a hospital in southern Davao city due to hypertension, local military spokesman Maj. Randolph Cabangbang said. The hospital was surrounded by soldiers and police.

Fully armed soldiers and police manning road checkpoints in Maguindanao stopped all vehicles and ordered occupants to get out as they checked identification.

Dozens of jittery villagers near Shariff Aguak left their homes and headed to a nearby province with their children and some personal belongings loaded onto trucks, small vans and tractors.