-LRB- CNN -RRB- -- 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 . Group : Philippines most dangerous country for journalists 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 .