MANILA, April 18, 2010 (AFP) - The families of those killed in the Philippines' worst political massacre vowed on Sunday to appeal a government decision to drop charges against two members of a Muslim clan blamed for the murders.

"We are going to oppose this order in court. We will file an appeal," Harry Roque, a private prosecutor representing some of the families, told AFP.

"They are angry, feel they were absolutely betrayed by the government, and believe there is a political compromise," he charged.

Justice Secretary Alberto Agra on Saturday ordered murder charges be dropped against cousins Zaldy and Akmad Ampatuan for the November massacre in strife-torn Maguindanao province.

He said both men presented alibis that showed they were not present at the time of the massacre in which 57 people, including many journalists, were executed at close range by gunmen loyal to the Ampatuans.

Government prosecutors had initially charged that both men and other members of the Ampatuan clan conspired to carry out the murders.

The murders were allegedly intended to prevent a rival from running against clan member Andal Ampatuan Jnr for the post of provincial governor in the May 10 national elections.

"The decision dropping murder charges against them was so sly and done on a weekend when no one was looking," Roque said. "It is so brazen, and shows that justice will not be reached in this administration."

Roque said his clients would also ask the court handling the case to disqualify the present panel of state prosecutors and appoint new ones.

The move also drew condemnation from US-based Human Rights Watch, which questioned the commitment of President Gloria Arroyo to see justice done in the case.

"President Arroyo's promise of justice for the killings is rendered mere lip service," said Elaine Pearson, the watchdog's deputy Asia director.

The Ampatuan clan had enjoyed close political ties with Arroyo until she was forced to disown them amid international outrage in the aftermath of the massacre.

Arroyo's government had in the past used the clan's huge private army to help the government in its anti-insurgency drive, and to ensure that her candidates won in the south during elections, security analysts said.

In exchange, the government allegedly turned a blind eye to corruption and abuses in the local government, while the clan was building up its vast wealth.

Arroyo's spokesman, Gary Olivar, on Sunday sought to shield the president from the decision, amid accusations by critics she may have intervened.

"It is obscene to insinuate that the president would interfere in the legal proceedings of a heinous crime like the Maguindanao massacre as a mere political accommodation to anyone," Olivar said in a statement.