The declaration is the first time martial law has been imposed anywhere in the Philippines since late dictator Ferdinand Marcos declared it nationwide more than 30 years ago.

Speaking to Al Jazeera on Monday, Harry Roque, a human rights lawyer said the legal challenge filed with the Philippines supreme court "questioned the constitutionality of the presidents declaration".

IN DEPTH


 Profile: Andal Ampatuan Jr
 Witness: 'We just followed orders'

He said the current constitution, drafted following the Marcos dictatorship, clearly limited the declaration of martial law to circumstances "when there is a foreign invasion or when there is an actual taking up of arms for the purpose of overthrowing the Philippine government".

But the situation in Maguindanao, Roque said, was one of lawlessness and not one that fitted with the constitutional requirements for imposing military rule.

"The task at hand now is to effectively investigate and prosecute those behind this dastardly act, rather than declare martial law," he said.

Ambush

Government prosecutors have named several senior members of the Ampatuan clan as the key suspects in the November 23 massacre.

Thousands of troops and police have been deployed to the province [AFP]
The killings followed an ambush allegedly carried out by gunmen loyal to the Ampatuans against a convoy of rival political activists and accompanying journalists.

Among those arrested in connection with the killings are Andal Ampatuan Sr, the clan's patriarch and former provincial governor, as well as at least six other family members and about 60 followers.

His son, Andal Ampatuan Jr, who surrendered to police shortly after the killings, is so far the only one to have been charged with multiple counts of murder.

Prosecutors have said further murder charges will follow shortly, but the Ampatuans have repeatedly denied any involvement.

Military raids

In the wake of the massacre, the Philippines government ordered thousands of troops to the province, surrounding several compounds owned by the Ampatuan family.

Military officials say a series of raids have uncovered a vast cache of weapons and ammunition stored by the family and their supporters.

Commanders overseeing the operation have said they are hunting up to 4,000 followers of the Ampatuans, who they believed are heavily armed and capable of carrying out bombings, arson attacks and abductions.

Security forces have sealed off Maguindanao's exit points and mounted dozens of checkpoints across the province, Andres Caro, operations director for the Philippines' national police, told reporters.