President Gloria Arroyo has imposed martial law in the Philippines province of Maguindanao after the election-related massacre of 57 people.Authorities also took the head of a powerful clan and several other members of the family into custody for questioning over the killings.
Troops raided the family's compounds on Friday, finding a buried arsenal.
It is the first time martial law has been used in the country since the fall of autocrat Ferdinand Marcos in 1986.
The favourite for next year's presidential elections, Senator Benigno Aquino, was quoted as opposing the move.
"We believe there's no basis for the implementation of martial law," he said.
Officials said they acted after receiving reports that armed groups loyal to the Ampatuan clan were planning an insurrection.
Among those taken into custody were Andal Ampatuan Sr, a powerful local official, and one of his sons, Zaldy - governor of the Autonomous Region in Muslim Mindanao, which includes Maguindanao.
Another son, Andal Ampatuan Jr, has been charged with 25 counts of murder and has so far been the main suspect in the case.
Arroyo legacy at risk in massacre
Prosecutors have issued subpoenas to six members of the Ampatuan family, including Andal Ampatuan Sr, to appear at a hearing on 18 December.
They deny any involvement in the 23 November attack on a political rival's convoy and have not yet been charged.
Among the 57 victims were members of a rival clan and 30 journalists.
Presidential spokesman Cerge Remonde told reporters that President Arroyo had "taken this bold step [of declaring martial law in the province] in answer to the cry for justice of the [relatives of the] victims of the Maguindanao massacre".
The BBC's Rachel Harvey says this is a politically sensitive move by President Arroyo.
The government said it had intelligence that armed groups were plotting an offensive.
"We felt that this is a very imminent threat, so we recommended this proclamation," said armed forces head Gen Victor Ibrado.
"By their sheer number, they are really a threat to the peace and order of the province."
The deputy chief of staff, Maj Gen Gaudencio Pangilinan, said the military needed to help the area "return to normalcy as soon as we can".
"Right now there is a complete breakdown of the function of the government in the area," he said.
Local mayor Andal Ampatuan Jr faces 25 counts of murder
The Ampatuan clan have long controlled Maguindanao province and have been loyal supporters of President Arroyo.
Since the killings, Mr Ampatuan Sr and other members of the family have been expelled from her party.
Friday's raids followed the discovery of what a military spokesman described as an arsenal of weapons buried several hundred of metres outside the Ampatuan compound in Maguindanao's capital, Shariff Aguak.
"We estimate that these weapons are enough to arm a battalion [500 soldiers]," national police chief Jesus Verzosa said.
An investigation has been ordered into how the military weapons ended up in the hands of the Ampatuans.
The government has armed militias in the south to act as an auxiliary force to the army and police battling insurgents but they often end up as the private armies of local strongmen.
Former Philippine leader Ferdinand Marcos imposed martial law nationwide from 1972 until 1981. He stayed in power until 1986.
