MANILA, Philippines ? (UPDATE) President Gloria Macapagal-Arroyo declared martial law in a southern province to quell a rebellion by forces there loyal to a powerful clan accused of a massacre, the government said Saturday.

?We noticed and observed there was a rebellion in the offing,? Justice Secretary Agnes Devanadera told a nationally televised news conference as she outlined the reasons for the imposition of martial law in Maguindanao province.

"The local governments in the whole of Maguindanao had been removed from the legitimate authorities. Some of these local government units... had closed down.

Devanadera also noted that government services in the province were no longer functioning, such as the judiciary system.

She noted that several prosecutors in Maguindanao based in Cotabato City had gone on leave.

?What we?re saying is that hindi na sinusunod yung [they are no longer following the] legitimate government authorities and dapat bukas yung lahat ng tanggapan ng pamahalaan pero dun sa nakita natin ngayon, hindi na sinusunod yan, sarado na [that all government offices should be open but from what we?ve seen that is no longer followed, these are closed], which is practically removing from the legitimate constituted authority the allegiance of these units,? she said.

?In effect, what?s happening there in Maguindanao is that there are groups of people who are taking their leadership to remove the allegiance of the people from the duly-constituted government. Dahil sa kanilang pananakot ay hindi na talaga pumapasok yung tao so hindi na talaga sinusunod ang batas [Because of the fear being sowed, people no longer go to work, the law is no longer followed]. Hindi na sinusunod ang duly-constituted government, like the courts [The duly-constituted government like the courts is no longer followed],? Devanadera added.

She explained that because of the absence of judges in the province, they had to bring in judges from other areas like Sultan Kudarat and Koronadal City in South Cotabato.

In the same news conference, Brigadier General Gaudencio Pangilinan said that they have intelligence reports indicating that several armed groups consisting of around 20 to 100 ?heavily armed? men were amassing in eight municipalities in Maguindanao purportedly to commit violent acts there.

?We have consistent reports that our intelligence has confirmed the presence of small groups of, perhaps supporters of the governor, armed supporters in the vicinities of several towns. We have identified at least eight spots where there are at least 50 [armed men]; one spot could number more than a hundred; the others are in small groups of 20. These are all potential problems,? Pangilinan said.

?There were indications that violence is imminent there. The capability to do it is there so we had to do something,? he added.

Meanwhile, police are continuously putting under their custody members of the influential Ampatuan clan, including some ranking government officials in the area to determine their connection with the killings and even the rebellion.

Police Director for Operations Andres Caro II said that at this time, seven people have been put under police custody: Maguindanao Governor Andal Ampatuan Sr., Autonomous Region of Muslim Mindanao Governor Zaldy Ampatuan, Mamasapano town Mayor Akmad Ampatuan, Shariff Aguak Mayor Anwar Ampatuan, several local government officials in Shariff Aguak namely Paisal Sullai and Cahoiner Ampatuan, and Kaiser Usman, the Agriculture secretary in ARMM.

Arroyo placed Maguindanao province under military control late on Friday, in one of the most dramatic developments following the massacre of 57 people there on November 23.

A local mayor, Andal Ampatuan Jr., has been charged with 25 counts of murder over the killings.

Police allege Ampatuan Jr. and 100 of his gunmen shot dead the occupants of a convoy that included relatives of his rival for the post of Maguindanao governor in next year's elections, as well as a group of journalists.

The rival, Esmael Mangudadatu, said the killings were carried out to stop him from running for office.

Ampatuan Jr. is the son of the province's governor, Andal Ampatuan Sr., who has ruled the province with the backing of a private army since 2001.

Within hours of martial law being declared, special forces detained Ampatuan Sr., the clan patriarch and three other members of the family who held senior government posts in the province.

Police had filed an indictment against Ampatuan Sr. alleging he was involved in the massacre, and were waiting on the justice department to decide whether to file charges.