( CNN ) -- The Philippine House of Representatives and Senate were meeting Monday to debate the imposition of martial law in the country 's south by President Gloria Macapagal-Arroyo in the aftermath of last month 's massacre of 57 civilians . As required by law , the president issued a report Sunday explaining her reasons for making the proclamation for the province of Maguindanao . `` Lawless elements have taken up arms and committed public uprising against the duly constituted government and against the people of Maguindanao , '' Arroyo said in a 20-page letter to the leaders of the House and Senate . Martial law went into effect Friday night , allowing arrests without a warrant . Army spokesman Lt. Col. Romeo Brawner Jr. said it was necessary to impose peace following what has been called a politically motivated massacre . He added that Congress would have to approve any extension beyond 60 days . The House and Senate might jointly convene on Tuesday to discuss the president 's report , House Speaker Prospero Nograles said Sunday , according to the Philippine News Agency . The debate began as elite forces of the Philippine National Police clashed in the town of Datu Unsay with suspected followers of the politically powerful Ampatuan family , which has been implicated in the massacre , the Philippine News Agency reported Monday . Reports said the armed men offered stiff resistance on Sunday against government security forces in a 10-minute firefight , withdrawing after government reinforcements arrived . No casualties were reported on either side . Charges of rebellion will be leveled against many of those arrested since martial law was implemented , Philippine Justice Secretary Agnes Devanadera said Saturday , CNN affiliate ABS-CBN reported . 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 . '' 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 . 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 .
Arroyo outlines reasons for martial law declaration in 20-page letter . Report : Arroyo , Senate might jointly convene Monday , House Speaker says . at least one members of Muslim family arrested , ABS-CBN reports . Authorities raid At least six warehouse and ranch belonging to family .