Philippine President Gloria Macapagal Arroyo declared martial law in parts of the southern Philippines Saturday as troops detained the patriarch of a powerful political clan in connection with the massacre of 57 political rivals and journalists last month.

Armed forces chief Gen. Victor Ibrado said clan leader Andal Ampatuan Sr. – a former governor of Maguindanao province where the killings occurred – was taken into custody for questioning Saturday morning along with at least six other family members. His son, Andal Ampatuan...