Philippines President Gloria Arroyo has vowed to "wipe out" the separatist group which said it planted a trio of bombs that left at least 11 dead.
Security has been stepped up in public places since the bombings
Security has been stepped up at public sites across the country amid fears the blasts may be part of a wider campaign.
About 130 were hurt in the blasts, one in Manila and two in the south.
The separatist group Abu Sayyaf, which is battling troops on the island of Jolo, claimed it planted the bombs and warned of more attacks.
A rebel spokesman reportedly told local radio that Monday's attacks were a "Valentine's gift" to Mrs Arroyo.
The president called on the public to unite behind the fight against terror.
"More than ever, we must not pull back but move forward to wipe out the remnants of the Abu Sayyaf," she said in a statement.
"The evil of terrorism has only one aim. It is to rule with absolute power and absolute force."
The government on Tuesday offered a 500,000 peso ($9,260) reward for information leading to the arrest of the al-Qaeda-linked Abu Sayyaf militants, AFP said.
Army and police intelligence officials said they were not ruling out a role by Jemaah Islamiah, a regional network of militants which has been blamed for previous bomb attacks on public places.
Security forces have been placed on high alert around the Philippines' airports, ports, bus terminals, shopping malls and foreign embassies.
One of Monday's blasts happened in General Santos City, when a bomb destroyed a parked motorcycle taxi outside a shopping mall, killing at least three people.
Almost simultaneously, a bomb exploded at a bus terminal in Davao City. A 12-year-old boy is reported to have died in the attack.
About half an hour later, a third blast went off in the Makati business district of the capital, Manila, killing at least three people.
In a phone call to DZBB radio, Abu Sayyaf spokesman Abu Solaiman said the militant group was responsible and linked the attacks to fighting between government forces and rebels on Jolo island.
"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," Mr Solaiman said.
Abu Sayyaf, founded in the early 1990s, is waging a violent bombing and kidnapping campaign against the central government in Manila.
It is considered a terrorist organisation by the Philippines and the US.
The group has carried out a series of kidnappings of Western nationals and has also been blamed for the bombing of a passenger ferry in Manila Bay in February 2004, which resulted in the death of more than 100 people.