MANILA, Philippines —  Three bombs jolted Manila and two other Philippine cities Monday, killing at least nine people and wounding more than 100 others, police said. The Muslim extremist group Abu Sayyaf (search) claimed responsibility for the blasts.

The group said the bombings were retribution for a major military offensive against Islamic militants in the southern Philippines, where 60 people have been killed in recent clashes.

"You can attribute this to us," an Abu Sayyaf leader, Abu Solaiman (search), told DZBB radio 20 minutes after Monday's first two blasts.

"There is one more to come," he said before the Manila bombing.

In a second telephone call, Solaiman said the bombings were a Valentine's Day "gift" to President Gloria Macapagal Arroyo (search).

National police head Gen. Edgar Aglipay ordered the 114,000-member police force to tighten security in vital installations and commercial establishments nationwide.

"These are despicable acts of terror, and we ask the people to brace themselves against these attacks on our freedom and security," presidential spokesman Ignacio Bunye said.

"We're not going to sleep tonight," added national security adviser Norberto Gonzales.

The Manila bombing occurred about 7:30 p.m. on a bus along a busy highway just below a station for an elevated train and near the Intercontinental Hotel. Police said three people were killed and at least 60 others injured as the bus exploded in flames and sent debris that hit two other buses.

The back of the bus was destroyed by the blast and all of its windows were blown out. The charred body of one passenger was still seated with a hand holding the seat railing. President Arroyo later visited the site.

A blast outside the Gaisano Mall in southern General Santos city about an hour earlier could be heard one mile away, witnesses said. Police said at least five people were killed and at least 36 others injured.

National police chief Edgar Aglipay said the bomb was believed to have been placed in a bag at a stand for three-wheel taxis about 30 yards from the mall entrance.

Another bomb, believed to have been made from a mortar shell, killed a 12-year-old boy and injured at least eight people when it went off almost simultaneously at a bus terminal in Davao, also in the southern Philippines, home to a restive Muslim minority, terror groups and criminal gangs. Mayor Rodrigo called it "the handiwork of terrorists."

Abu Sayyaf leader Solaiman promised more attacks in the future.

"Our latest operations — planned and executed with precision by the gallant warriors of Islam — is our continuing response to the Philippine government's atrocities committed against Muslims everywhere," Solaiman said.

"We will find more ways and means to inflict more harm to your people's lives and properties, and we will not stop unless we get justice for the countless Muslims lives and properties that you people have destroyed."

Officials had expressed concerns over the prospect of a terrorist attack as the military carries out an all-out assault on Jolo island (search) against a group of gunmen who recently attacked troops in the region, sparking clashes that have killed at least 60.

The gunmen are believed to include followers of jailed Muslim leader Nur Misuari (search), backed by Abu Sayyaf members.