Government soldiers rush forward together with an armoured personnel carrier to give assistance to their comrades in pursuit of unidentified armed men on Monday, Dec. 7, 2009 in Maguindanao province in southern Philippines. (Pat Roque/Associated Press)

Gunmen loyal to a clan accused in the Philippines' worst political massacre clashed with police in the first reported fighting since martial law was imposed in the region over the weekend.

No casualties were reported after the incident late Sunday night, in which 20 to 30 armed followers of the Ampatuan clan opened fire on police patrolling Datu Unsay township in the province of Maguindanao, national police chief Jesus Verzosa said Monday.

The Ampatuans are the prime suspects in the Nov. 23 massacre of 57 people who were travelling with an election caravan of a rival candidate for governor in the province.

The clash comes as government negotiators attempt to persuade gunmen loyal to the Ampatuans to surrender peacefully and avoid bloodshed, Verzosa told reporters.

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.

Police over the weekend raided Ampatuan mansions, farms and warehouses and discovered huge caches of weapons, including machine-guns and grenade launchers.

Martial law decree faces opposition

The clash will likely be used as ammunition for President Gloria Macapagal Arroyo in her efforts to maintain the martial law order imposed on the province on Friday, the first use of military rule in the Philippines since the late dictator Ferdinand Marcos declared it nationwide 30 years ago.

A group of human rights lawyers challenged the martial law order on Monday, asking the Supreme Court to overturn the order on the grounds the law and order breakdown in Maguindanao did not amount to a rebellion.

An Ampatuan ally, Rep. Didagen Dilangalen, also filed a separate motion against martial law.

On Sunday Arroyo sent her report on the martial law declaration to the Philippines Congress, which will convene on Tuesday to approve or reject it.

The political crisis in the country began when a caravan of vehicles, being followed by more than a dozen journalists, was on its way to register Ismael Mangudadatu as a candidate for governor in Maguindanao, which is located on the country's southern island of Mindanao.

The caravan and journalists were reportedly stopped by 100 armed men and taken at gunpoint to a grassy hill, where they were shot or butchered with machetes.

More charges pending

Authorities charged Andal Ampatuan Jr., a town mayor in the province and the son of current provincial governor Datu Andal Ampatuan Sr., on allegations he had a role in the killings. Prosecutors are also planning to file rebellion charges against the elder Ampatuan and more than 20 other people, Justice Secretary Agnes Devanadera said.

The Ampatuan clan had ruled the region unopposed for years and has a large private army. They were also, prior to the crisis, political allies of Arroyo and members of the ruling Lakas Kampi CMD party and helped deliver crucial votes from the region during the 2004 election.

Arroyo, facing local and international pressure to crack down on regional warlords, expelled both members of the Ampatuan family plus Andal Ampatuan Jr.'s brother Zaldy, the governor of the Autonomous Region in Muslim Mindanao, from the party.

The Ampatuans have denied any responsibility, accusing Muslim separatist guerrillas of staging the attacks.

With files from The Associated Press