MANILA — A powerful explosion ripped through three floors of a shopping mall in the heart of Manila’s financial district today, killing eight people, injuring scores and sending police and troops on the highest state of alert.

President Arroyo said “circumstances indicate it is highly probable that it was caused by an explosive device.” The same mall was targeted by a bombing in 2000.

She said police and the military went on the highest alert and deployed an additional 2,000 personnel to secure public areas “to prevent a similar occurrence.”

The afternoon explosion at the glitzy Glorietta 2 mall toppled roofs, destroyed walls, and sent debris crashing onto cars outside.

At least eight people were killed and more than 70 were injured, a Manila police chief, Geary Barias, said.

Police sealed off the shopping complex and searched debris for clues of explosives.

National Security Adviser Norberto Gonzales said authorities were looking into a possible terrorist attack, but “there is no conclusive evidence yet.”

A taxi driver, Mario Em, said he had just dropped two women off at the mall when the blast hurled the passengers against his vehicle, killing them instantly.

He said he pulled one of the victims, who was pregnant, from underneath his car.

Officials said the shock waves from the blast, which appeared to have originated from a basement entry for delivery vehicles, shot through three floors of the mall as well as sideways.

Mae Ann Sison said her sister, Angelica Cortez, was on an escalator going down from the second floor when the blast tossed her in the air.

“She landed on the escalator and her right foot got caught in the escalator chain and she was hit by glass shards from shops around her,” Ms. Sison said, adding a chunk of concrete hit her sister’s head.

Al Qaeda-linked militants, who have waged a yearslong bloody bombing campaign in the southern Philippines, have targeted Manila before.