(CNN) -- Philippines President Gloria Macapagal-Arroyo declared martial law beginning Friday night in the southern province of Maguindanao, where 57 people were killed last week, an army spokesman told CNN Saturday.
Army spokesman Lt. Col. Romeo Brawner Jr. said martial law -- under which police can make arrests without a warrant -- went into effect at 9 p.m. Congress would have to approve any extension beyond 60 days.
Brawner said the measure was invoked to impose peace on the region after the unrest, which led to troops being mobilized.
"Life is to resume normally and martial law is established to prevent new violence, with troops on the ground monitoring any possible violence breakout," he said. "But the civilian government will be running the affairs of the province, not a military governor."
Military police have detained three brothers and their father on suspicion of involvement in the unrest: Akmad Ampatuan, the vice governor of Maguindanao; Anwar Ampatuan, the mayor of Sherif Auguak, Maguindanao's capital; and Zaldy Ampatuan, governor of the Autonomous Region of Muslim Mindanao.
Andal Ampatuan Sr., the patriarch, became ill and was hospitalized after he too was taken in.
"Local government will function, it will prevent armed groups from wreaking havoc ... and it will allow the officials to search and find the guilty parties," Brawner said.
Security forces on Friday searched the home of Andal Ampatuan Jr., mayor of Datu Unsay and son of the provincial governor of Maguindanao, according to Brawner.
Authorities say Andal Ampatuan Jr. directed the November 23 politically motivated killings. Weapons were found in the house, Brawner said.
On Tuesday, Philippine authorities charged Ampatuan and other suspects with 25 counts of murder in the killings. Still more suspects were taken into custody Friday.
The Philippine military is investigating its own forces in connection with the case as well, Brawner said.
The National Bureau of Investigation has moved a third witness to Manila for questioning, the Philippines News Agency reported Thursday.
"At the moment, we are interviewing the witness, so that we would know what he knows about the incident in Maguindanao," NBI Director Nestor Mantaring said. The witness claimed to have been at the site during the massacre.
"He is very vital," Mantaring said. "From what I know ... he knows a lot of information regarding the incident."
Among the victims in the massacre were 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, the father of the accused mayor, saying he would be kidnapped if he filed the papers himself.
Witnesses and local officials have blamed the younger Ampatuan, a longtime ally of the Philippines president and a known warlord. They said the killings were an attempt to block Mangudadatu from challenging him in the May gubernatorial election.
Also killed were a dozen journalists who had accompanied the women.
Violence in the run-up to elections is not uncommon in the Philippines. The Maguindanao massacre, however, is the worst politically motivated violence in recent Philippine history, according to state media.
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.
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.
CNN's Talia Kayali contributed to this report.