President Gloria Arroyo's critics and human rights groups have criticised the declaration of rebellion because it gives a political gloss to the crimes and allows suspects to be granted bail and presidential amnesty.
Mrs Arroyo also faces legal action by lawyers and civil society groups for her declaration of martial law in Maguindanao.
The South China Morning Post newspaper said the Senate defence committee chairman Rodolfo Biazon, a former army chief of staff, was one of the dissenting voices.
"I am disconcerted when they say they are going to file rebellion charges against the Ampatuans. That to me is the first step to absolving the Ampatuans of mass murder charges," he was quoted as saying.
A variety of conspiracy theories has been put forward for the declaration of martial law just months ahead of elections.
The vice-governor of North Cotabato province, Manny Pinol, was quoted as saying he had received reports of the Ampatuans threatening to "tell all they knew about electoral cheating" if Mrs Arroyo abandoned them.
The clan has long controlled Maguindanao and its members have been loyal supporters of President Arroyo.
Former Maguindanao governor Andal Ampatuan Sr, and four of his sons, were among suspects held last week.
One of the sons, Andal Ampatuan Jr, is suspected of orchestrating the 23 November massacre and has been charged with 25 counts of murder.
It is the first time martial law has been used in the country since the fall of autocrat Ferdinand Marcos in 1986.
Marcos imposed martial law nationwide from 1972 until 1981. He stayed in power until 1986.
via BBC
