Gunmen loyal to a powerful clan accused in the Philippines' worst political massacre clashed with police in the first reported violence since a southern province came under martial law, officials said Monday.

No casualties were reported, and government negotiatiors were trying to persuade about 2,400 gunmen to surrender peacefully, 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 Sunday while they were patrolling Datu Unsay township, near the site of the massacre, said national police chief Jesus Verzosa.

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 up in 16 of Maguindanao's 22 townships to defend the Ampatuan clan, which has ruled the province unopposed for years.

President Gloria Macapagal Arroyo imposed martial law in the predominantly Muslim province late Friday. Officials say the Ampatuans and their followers were planning rebellion. At the weekend, authorities arrested the family patriarch and at least six other family members and about 60 followers.

Civil rights groups were set to challenge at the Supreme Court Arroyo's martial law declaration, which authorized thousands of troops to make arrests without court warrants and crack down on the Ampatuans and their private army.