Also Monday, a mayor was assassinated, shortly after leading a flag-raising ceremony at town hall. Authorities investigating the shooting of Mayor Jesus Sebastian Jr. of Ilagan - involving four men wearing military uniforms - say communist rebels have claimed responsibility.
President Gloria Macapagal Arroyo called the series of bombs that struck the city of General Santos on Sunday "a crime against the Filipino people." The worst blast killed at least 14 people outside a busy department store and injured 69 others. Police said they arrested two suspects Monday.
"This evil will not go unpunished," Arroyo said in a statement issued hours after visiting the predominantly Christian city and meeting with its mayor. "We will fight terrorism to its end."
Arroyo's declaration came as 2,700 Americans and 2,900 Filipinos launched a three-week joint military exercise on the island of Luzon in the northern Philippines that will focus on war games and civil projects.
Anti-terrorism has been a key focus of the U.S. training exercises with Filipino forces.
Arroyo said federal officials would work with local leaders to impose curfews, increase checkpoints and enhance other security measures. She urged the Philippine congress to pass a pending anti-terrorism bill that would raise penalties for violators and increase police powers.
"The whole community must be involved in this effort," she said. "I call on our people to be vigilant. As we decisively move forward in the campaign against terrorists, they will tend to be more desperate."
As Arroyo met with Mayor Pedro Acharon, a small bomb exploded on a fishing boat in an outlying village. No injuries were reported.
A man who called a radio station to warn of Sunday's explosions called back Monday, saying they were retribution for alleged attacks against Muslim civilians by the Philippine military in its U.S.-backed war against Abu Sayyaf rebels.
The man identified himself as Abu Muslim al-Ghazie and said he represented the Abu Sayyaf. In a call to the Radio Mindanao Network before the blasts, he claimed 18 bombs had been planted around the city.
"In our fight against the government, we cannot deny the fact that civilians will be in the cross fire," the man told the station Monday.
Other Abu Sayyaf spokesmen said they had no knowledge of the group's involvement.
Police have blamed unspecified "terrorists." They arrested two suspects who they said belonged to the Muslim separatist Moro Islamic Liberation Front, and confiscated guns and a grenade in a pre-dawn raid Monday.
The Abu Sayyaf, believed to have ties to Osama bin Laden's al Qaeda network, and the fundamentalist Moro rebels have been blamed for setting off bombs in General Santos in the past.
The city is about 130 miles from Basilan island, where the Abu Sayyaf has been holding a U.S. missionary couple and a Filipino nurse hostage for nearly 11 months. About 160 U.S. Special Forces troops are on the island on a six-month counterterrorism training mission aimed at helping the Philippine military crush the guerrillas, who have beheaded other hostages, including an American.
The first bomb exploded Sunday in a three-wheel motorcycle taxi parked in front of a department store in the business district of General Santos, about 630 miles southeast of Manila. Four of the dead were children.
Within 40 minutes, two other bombs went off elsewhere in the city. Most businesses closed and checkpoints were set up on major roads.
The scene was reminiscent of nearly simultaneous bombings in Manila 16 months ago that killed 22 people. An Indonesian man who claimed he planned those blasts pleaded guilty Thursday in General Santos to explosives possession after leading police to a buried cache of more than a ton of TNT, detonating cords and M-16 rifles in January.