"This is definitely an act of terrorism, although we have not yet pinpointed what group is responsible for it," National Security Adviser Norberto Gonzales said.

Muslim insurgents in the southern Philippines frequently bomb civilian targets, but such attacks are rare in Manila. Authorities citing intelligence reports recently had warned that terrorists were plotting attacks in the capital.

President Gloria Macapagal Arroyo called for national unity and urged Filipinos to be on the lookout for further attacks. Arroyo ordered police and the armed forces to be on the "highest alert," and 2,000 extra police were deployed in the capital.

As medical teams treated scores of wounded, three nearby hospitals reportedly received death threats.

"I warn those who seek to exploit this incident to destabilize our government for their selfish political motives," Arroyo said in a televised address from Malacanang Palace.

Gabriela Women's Party, one of numerous groups pressuring Arroyo to step down over alleged corruption, pointed the finger of suspicion for the blast at her government.

The group issued a statement "to express our alarm and suspicion over the impeccable timing of the incident in the light of snowballing calls for Mrs. Arroyo to resign."

The explosion at an entrance to Glorieta Mall, in Manila's financial district, occurred when the mall was crowded with lunchtime shoppers. The mall, in Makati, is near several five-star hotels popular with foreign tourists and business people.

The blast caused extensive damage, spraying pedestrians, cars and trucks with shattered glass, large hunks of concrete and strips of metal. It also left a large crater in the floor.