The British broadcaster said the move was accompanied by the arrest of the head of a powerful local political clan and several other family members as troops staged raids on the group's compounds Friday.
Officials said powerful Maguindanao leader Andal Ampatuan Sr., and one of his sons, Zaldy, the governor of the Autonomous Region in Muslim Mindanao, were arrested for questioning in connection with the Nov. 23 attack on a political rival's convoy.
Andal Ampatuan Jr., had already been charged with 25 counts of murder in the case. Philippines officials said martial law was declared because they feared the armed groups backing the clan were preparing an armed insurrection.
"We felt that this is a very imminent threat, so we recommended this proclamation," armed forces head Gen. Victor Ibrado told the BBC.