MANILA, Philippines (AP) – A bomb exploded in a market packed with Christmas shoppers Sunday, killing at least 15 people, injuring 58 others and shattering a monthslong lull in terror attacks in the volatile southern Philippines, where Muslim and communist rebels are active.

The homemade bomb, concealed in a box, went off in the meat section of the market in General Santos, about 620 miles south of Manila.

Officials immediately bolstered security in the predominantly Christian port city of 500,000 people, fearing more attacks.

President Gloria Macapagal Arroyo said there was no way to justify “this heinous deed.”

No one claimed responsibility, and it was not yet clear whether terrorist groups were involved. Muslim and communist rebels both operate in areas around General Santos.

The city had been largely tranquil since a bomb killed 14 people in a shopping mall in 2002.

Authorities blamed the Muslim extremist group Abu Sayyaf and a larger separatist group, the Moro Islamic Liberation Front. Some of those arrested are being tried on multiple murder charges.

Police investigator Capt. Maximo Sebastian said three people were killed instantly by Sunday afternoon’s blast. Other victims died in hospitals.

“The market was packed with people because there were Christmas flea market stalls there, and the explosion was powerful,” Sebastian told The Associated Press by telephone.

Soldiers and police cordoned off the area, fearing more explosives may have been planted, said army Col. Medardo Geslani, who heads a regional anti-terrorism force. Security forces would boost their presence in public places, intensify patrols and set up checkpoints, he said.

Geslani said he considered the bombing a terror act and a reminder of the constant vigilance needed to thwart such attacks, even after long spells of relative peace.

“We know they have long-standing plans. We pre-empt them most of the time, but there is no impenetrable area. There are times they can get through, and this time they really did,” Geslani told AP.

Sen. Richard Gordon, who heads the Philippine Red Cross, criticized the military and police for failing to prevent the attack despite what he said was intelligence indicating a planned terror strike in the city.

“I’m getting reports from some of our people there that they knew there was a plan to pull this off but still it happened,” Gordon told ABS-CBN television. “They need to bolster their spying and their surveillance of places that should be under guard.”

Despite on-and-off crackdowns by the military and police, Muslim militants and communist guerrillas are believed to still have a presence in General Santos and other key cities in the country’s volatile south, home to minority Muslims.

The sprawling archipelago has suffered a rash of bloody terror attacks and mass kidnappings in recent years that largely have been blamed on Muslim extremists. Communist rebels have staged attacks on security forces, local officials and infrastructure such as electricity and mobile phone transmission towers.

The Philippines has been a key ally in the U.S. war on terrorism, with Washington providing extensive counterterrorism training and gear for the poorly equipped military.

The Abu Sayyaf, an Islamic organization that appears on a U.S. list of terror groups, claimed responsibility for a bomb that killed more than 100 people on a ferry leaving Manila in February. It continues to threaten attacks despite battle setbacks and a reported lack of operational funds.