Gunmen loyal to a powerful clan accused in the Philippines' worst political massacre clashed with police in the first reported violence since martial law was imposed in the southern region.
No casualties were reported and government negotiators were trying to persuade about 2,400 gunmen to surrender peacefully to avoid bloodshed, Interior Secretary Ronaldo Puno told reporters in Manila.
About 20-30 armed followers of the Ampatuan clan, the main suspect in the Nov. 23 massacre of 57 people traveling in a rival's election convoy, opened fire on police commandos late Sunday while they were patrolling Datu Unsay township, near the site of the massacre, national police chief Jesus Verzosa said.
The attackers withdrew after two armored troop carriers were sent to repel them, Verzosa said.
He said the assailants were among more than 2,400 gunmen who had massed in 16 of Maguindanao's 36 townships to defend the Ampatuan clan, which has ruled the province unopposed for years.
Government negotiators were trying to convince the gunmen to surrender to avoid bloodshed that could harm civilians, Verzosa said.
"If they don't, we'll have to look for them," Verzosa told foreign correspondents in Manila.
Army troops have raided the Ampatuans' mansions, farms, warehouses and offices and discovered a huge arsenal of weapons, including machine guns, grenade launchers and nearly half a million rounds of ammunition.
An anti-money laundering agency was assessing whether the Ampatuans, who have built sprawling mansions in a region notorious for appalling poverty, have acquired wealth illegally so those assets can be frozen, Justice Secretary Agnes Devanadera said.
The latest crisis to hit President Gloria Macapagal Arroyo's tumultuous reign started Nov. 23, when a convoy led by a rival politician was stopped by about 100 gunmen allegedly led by Datu Unsay Mayor Andal Ampatuan Jr. The 57 people in the convoy, including 30 journalists, were killed with guns, machetes and a backhoe.
Ampatuan, who is detained in Manila, has been charged with multiple counts of murder along with other clan members. Prosecutors also plan to file rebellion charges against the clan's patriarch and more than 20 other people on Tuesday, Devanadera said.
The Ampatuan clan has ruled the region with an iron fist for years and has a large private army. They have been regarded by many as untouchable warlords because of their political alliance with Arroyo.
The clan helped Arroyo win crucial votes from Maguindanao during 2004 election.
Amid an international outcry over the carnage and reported massing of Ampatuan's supporters, Arroyo on late Friday imposed martial law in Maguindanao _ the first use of military rule in the Philippines since late dictator Ferdinand Marcos declared it nationwide more than 30 years ago.
A group of lawyers asked the Supreme Court on Monday to stop the martial law imposition, arguing the law and order breakdown in Maguindanao did not amount to a rebellion.
An Ampatuan ally, Rep. Didagen Dilangalen, filed a separate motion against martial law.
Arroyo sent a report on her martial law declaration to Congress, which will convene Tuesday to approve or reject it. Her allies dominate the lower house.
Associated Press writers Jim Gomez, Teresa Cerojano and Hrvoje Hranjski in Manila contributed to this report.