But her strong response came amid widespread questions over whether action will be put to her words. The murders are alleged to have been arranged by one of her political allies and the Philippines has a long-standing culture of impunity for political violence.
"The government definitely has the numbers but not the political will," says Vilnor Papa, Philippine campaign manager of Amnesty International. "We have political killings. We have summary executions. This is a culture of impunity. You have seen how people from the military have gotten away with murder."
But while political murders are common in the Philippines, this was an extraordinary outrage – a brazen daylight attack in front of witnesses and with apparently no threat from police. At least a dozen of the victims were local journalists.
The killers were alleged, according to eyewitness reports broadcast on Philippine radio and television stations, to be under orders from Andal Ampatuan Jr., mayor of a town that bears his family name. The victims were stopped in Ampatuan, herded several miles away, and shot.
The victims had been on their way to enter the name of Ismael Mangudadatu, the vice mayor of another town, as a candidate for governor. Mr. Ampatuan is vying to succeed his father, Andal Sr., as governor of the province of Maguindanao. Mr. Mangudadatu was not among the victims.
The Ampatuan family are local warlords accustomed to winning elections, in part thanks to their close relationship with Arroyo and the military. The family helped her win the vast majority of votes in the province in the 2004 presidential election. Arroyo won 100 percent of the votes in some towns in the province and at some polling stations the number of votes for Arroyo exceeded the number of registered voters.
On a broader level, the massacre reflected the constant struggles among factions and families. In a region that was barely subdued by the Spanish and then the Americans, warlordism has replaced feudalism, with different groups fighting for political perks and payoffs.
Ampatuan's gubernatorial rival Mangudadatu is the doyen of another powerful local family and had refused to stand down amid reported threats against his life. Thinking gunmen would be reluctant to shoot women and journalists, he sent them in a convoy to register his candidacy. His wife and two sisters were among the dead.
Arroyo's government Tuesday took pains to distance itself from the Ampatuans, denouncing the killings as "unconscionable" and declaring that no one was "untouchable." The national police chief dismissed Provincial Police Chief Zukarno Adil Dicay and three other officers. National Police spokesman Leonardo Espina told local media that Mr. Dicay had been seen with some of the gunmen earlier on Monday, before the assault.
But strong words and highly publicized manhunts have become common after political killings in the Philippines, while successful prosecutions have been rare. Some analysts are convinced the Ampatuan family will remain in power.
The fact that the government has a large number of troops in the area adds to the mystique of impunity.
Arroyo has had to make compromises with Muslim family leaders throughout the region while soldiers pursued elements of the Moro Islamic Liberation Front, deeply ensconced in the province.
"The area is highly militarized," says Satur Ocampo, a longtime foe of the government who now serves as deputy leader of the opposition in the Philippines House of Representatives. "There's a high level of troops there. The Philippine National Police was on alert, but it's quite apparent they were unable to prevent this massacre."
Arroyo may face greater pressure to pursue justice in this case because of the uproar it has created. Since so many local journalists were killed, the government is under fire not only from the Philippines' National Union of Journalists but also from international organizations such as Reporters Without Borders and the Committee to Protect Journalists.
"There is pressure from all sectors for the government to do something," says Girlie Padilla, secretary general of the Ecumenical Movement for Justice and Peace.
Still, Ms. Padilla has doubts. "Will they cover up for the ally who did the killings?" she asks. "That would show they would do anything to cover up for this group. We will have a hard time getting justice."