Metro Manila police chief Director Edgar Aglipay refused to identify the groups for fear of jeopardizing ongoing investigations.
Meanwhile, President Arroyo assured the people of their safety during the Christmas holidays.
The bomb, fashioned out of some 20 kilograms of ammonium nitrate, fuel, wires and two alarm clocks, was safely detonated by police on Friday morning. The explosive was found at a construction site between the Allied Bank and Philippine Long Distance Telephone Co. buildings.
Hundreds of office workers as well as diplomats and the staff of the British and Canadian embassies were told to vacate their offices following the discovery of the explosive.
The Canadian Embassy is in the Allied Bank building while the British Embassy is located at the adjacent Locsin building.
Aglipay said fragments of the bomb, which was set off by a "small controlled explosion," may provide clues leading to the identity of the perpetrators.
Aglipay hinted that leftist groups may have planted the bomb to mark the anniversary of the Communist Party of the Phi-lippines, but did not rule out the involvement of rightist elements.
"All groups from the extreme left to the extreme right are being looked into, but there is a specific group that we are following up right now," Aglipay said.
A source in the police intelligence community said they were also eyeing the possible involvement of die-hard followers of Nur Misuari, former governor of the Autonomous Region in Muslim Mindanao, who staged a failed revolt in Zamboanga City on Nov. 19 in an apparent attempt to hold on to power.
Southern Police District director Chief Superintendent Jose Gutierrez Sr. said the bomb was made by specialists as indicated by the manner it was assembled.
Aglipay said the explosive was apparently intended to go off and not merely meant to cause confusion, but somehow, it malfunctioned.
Meanwhile, Aglipay ordered an increase in police patrols in shopping malls and other crowded areas in the metropolis to ward off any threats to public security.
Aglipay called on the people to immediately report to the authorities any suspicious-looking individuals or abandoned packages for prompt police action.
He also said he has created a task force headed by Chief Superintendent Rowland Albano that will secure vital installations in Metro Manila, including the oil depots in Pandacan.
Aglipay enjoined the five police districts in the metropolis to organize special teams that will provide heightened security measures in their respective jurisdictions.
Philippine National Police (PNP) chief Director General Leandro Mendoza has placed the entire PNP on high alert following discovery of the bomb in Makati to prevent a repetition of the Dec. 30 bomb attacks in Metro Manila that left 22 people killed and nearly 100 others wounded.
The Western Police District (WPD) said it has deployed additional policemen to protect devotees attending dawn Mass.
WPD spokesman Chief Inspector Gerry Agunod said the move was taken upon instructions of Manila Mayor Lito Atienza.
Presidential Spokesman Rigoberto Tiglao said the President had ordered the deployment of 3,000 additional policemen to secure some 20 shopping malls in Metro Manila.
Tiglao said most of the reinforcements are new recruits, some pulled out from the provinces, while others are office personnel with not much administrative work.
Tiglao also said the investigators were already following a "hot lead" in Friday’s failed bomb attack.
Mrs. Arroyo cited the police for acting swiftly after the bomb was discovered.
"Actually, we have anticipated that there will be those who will try to undertake Dec. 30 anniversary bombings," the President said.
She called on the plotters to "stop this cruel madness or suffer the wrath of our people."
"The President would like to assure our people that the police and intelligence units have not put down their guard, and will continue to be on alert so our people will enjoy the holidays in peace," Tiglao said.
The Dec. 30 blasts ripped through a coach of the Light Railway Transit in Blumentritt, Manila; shattered a passenger bus along EDSA in Cubao, Quezon City; and rocked an abandoned gasoline filling station on Ayala Avenue in Makati, a warehouse at the Ninoy Aquino International Airport in Parañaque City and Plaza Ferguson in Manila, across the US Embassy.
Police initially blamed urban guerrillas of the secessionist Moro Islamic Liberation Front (MILF) for last year’s bomb attacks, but later withdrew the accusation in the light of a forthcoming peace negotiations with the rebel group.
In another development, a mild confusion broke out at the Light Rail Transit station in Monumento, Caloocan City as police ordnance experts arrived to check on a black bag suspected to contain a bomb.
Operatives of the Special Weapons and Tactics (SWAT) in full battle gear came with bomb-sniffing dogs after an unidentified commuter reported the existence of the bag at the stairs leading to the boarding area.
The bag turned out to contain old clothes. — With reports from Marichu Villanueva, Jaime Laude, Mike Frialde, Jerry Botial