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.