(CNN) -- Charges of rebellion will be leveled against many of those arrested during martial law in the southern Philippines -- declared in the aftermath of last month's massacre of 57 civilians, the nation's justice secretary said.
Justice Secretary Agnes Devanadera made the statement Saturday as the administration of President Gloria Macapagal-Arroyo defended the martial law declaration, CNN affiliate ABS-CBN reported. Some lawmakers have challenged the legality of the declaration, which allows authorities to make arrests without a warrant.
Martial law went into effect Friday night in the province of Maguindanao to impose peace following the politically motivated massacre, said army spokesman Lt. Col. Romeo Brawner Jr. He added that Congress would have to approve any extension beyond 60 days.
Police and military reports of armed men massing in Maguindanao would lead to charges of rebellion, Devanadera said, according to ABS-CBN.
Signs indicated that "violence was imminent," said Brig. Gen. Gaudencio Pangilinan, operations chief of the Armed Forces of the Philippines, the affiliate reported.
The armed men were supporters of the politically powerful Ampatuan family, which has been implicated in the massacre, Pangilinan said.
At least six members of the Ampatuan family have been arrested, including Datu Unsay Mayor Andal Ampatuan Jr., according to ABS-CBN. Ampatuan, whose father is governor of Maguindanao, has been accused of directing the killings and has been charged with 25 counts of murder.
One of the massacre victims implicated members of the Ampatuan family before she died, the affiliate reported.
Over the weekend, authorities raided at least one warehouse and ranch belonging to the family. They confiscated firearms, ammunition and vehicles, Maj. Randolph Cabangbang, deputy of operations for the eastern Mindinao command, told CNN.
The military was looking at arresting at least 100 people tied to the massacre, ABS-CBN reported. The Philippine military is investigating its own forces in connection with the case as well, Brawner said.
Asked why martial law was imposed 12 days after the killings, Cabangbang said authorities "were trying to build a case, a tight case" against suspects.
"But it is taking long to build a case, so I think the government gave us a free hand in arresting those who are suspects, and allowed us to search, even without warrant. So we really need this declaration of state of martial law."
The president was to submit a written report to Congress on Sunday evening, justifying her declaration of martial law, the Department of Justice said, according to the state-run Philippine News Agency.
Political debate had flared over how soon Congress needed to convene after the declaration of martial law, as required by the Constitution.
The House of Representatives and the Senate might jointly convene on Tuesday to discuss the president's report, House Speaker Prospero Nograles said Sunday, according to the Philippine News Agency.
Violence in the run-up to elections is not uncommon in the country. The Maguindanao massacre, however, is the worst politically motivated violence in recent Philippine history, according to state media.
The victims included the wife and sister of political candidate Ismael "Toto" Mangudadatu, who had sent the women to file paperwork allowing him to run for governor of Maguindanao. He said he had received threats from allies of Gov. Andal Ampatuan Sr., the father of the accused mayor, saying he would be kidnapped if he filed the papers himself.
Ampatuan Sr. has been taken into custody in the massacre, but was hospitalized after taking ill.
Witnesses and local officials say the killings were an attempt to block Mangudadatu from challenging the younger Ampatuan -- a longtime ally of the Philippine president and a known warlord -- in the May gubernatorial election.
A dozen journalists who had accompanied the women were also killed in the massacre.
Suspicion fell on Ampatuan after a government construction vehicle was found at the hastily dug mass grave that held the bodies of the massacre victims.
Maguindanao is part of an autonomous region in predominantly Muslim Mindanao, which was set up in the 1990s to quell armed uprisings by people seeking an independent Muslim homeland in the predominantly Christian Asian nation.