Philippine police say they have identified and are seeking 161 people, including government militiamen, for involvement in last month's gruesome massacre of 57 people.

National police chief Jesus Verzosa says witnesses identified Andal Ampatuan Jr., a local mayor, as the person who led the killings.  He is accused of leading the group to stop supporters of a rival politician from registering him for next year's provincial governor elections in Maguindanao. 

Ampatuan Jr. has already been arrested and charged with 25 counts of murder.  About 30 of those suspected of involvement have also been arrested.

Ampatuan Jr.'s father, the governor of Maguindanao, and other members of his powerful clan have also been detained and may face murder charges.

The government has imposed martial law in the province to allow thousands of troops to make arrests without warrants.

It is the first time martial law has been used in the Philippines since dictator Ferdinand Marcos imposed it nationwide more than 30 years ago.

Critics have accused President Gloria Arroyo of overreacting, but her spokesman says the president had to take decisive action. He says once law and order is restored in the province, martial law will be lifted.

The military says the troops controlling Maguindanao province uncovered a large cache of weapons and ammunition Sunday on property owned by the Ampatuans.

Some information for this report was provided by AP and AFP.