Manila, Nov 25 - The Philippine government on Wednesday rejected mounting calls for the immediate arrest of members of a prominent political family allegedly behind the massacre of 46 people.

Presidential spokesman Cerge Remonde said investigators must build a strong case against the Ampatuan family, close allies of President Gloria Macapagal Arroyo, before any arrests could be made.

"We have due process to be observed also, so let us allow the investigators on the ground to come up with a case through their investigation," he said.

Remonde assured the public that all those behind the massacre would be arrested and prosecuted.

"It's a very big embarrassment, not only to the government, but also to the entire nation," he said.

The gruesome killings occurred on Monday in a hilly area in Salman village in Ampatuan town in Maguindanao province, 930 kilometres south of Manila.

The victims were on their way to file the certificate of candidacy of Buluan town Vice Mayor Esmael Mangudadatu for Maguindanao governor in next year's elections.

Mangudadatu's candidacy would pit him against the son of incumbent Governor Andal Ampatuan Senior, who had run unopposed in three previous elections.

The victims included Mangudadatu's wife, two sisters, two human rights lawyers and at least 17 local journalists who were covering the event.

They were shot multiple times at close range. Some of the bodies were buried in a shallow mass grave that was dug up by a backhoe of the provincial government.

Mangudadatu said his wife told him in a telephone call before she was killed that they were being held by 100 gunmen allegedly led by Mayor Andal Ampatuan Junior on orders of his father.

Commission on Human Rights chairwoman Leila De Lima questioned the "less-than-decisive action" of authorities in dealing with the worst-ever election-related violence in the country.

"There is an unusual treatment," she said. "If you're an ordinary suspect and there is a lead (on your alleged involvement in a crime), you will be immediately picked up by police."

"The police should at least invite the Ampatuans to explain their side in the precinct," she added.

The international community has condemned the murders, which United Nations Secretary General Ban Ki-moon called a "heinous crime".

Ban said he hoped "that no effort will be spared to bring justice and to hold the perpetrators accountable".

On Tuesday, Arroyo declared emergency rule in Maguindanao, the nearby province of Sultan Kudarat and Cotabato City to give security forces a free hand in going after the perpetrators.

But political analysts have expressed doubts Arroyo would come down hard on the Ampatuans, who have been key allies of the government especially in previous elections. (dpa)