The blast at the mall blew debris out into the street
The lunchtime blast shattered windows and sent debris crashing onto cars at the Glorietta shopping complex.
More than 100 people were hurt in the explosion, in the city's Makati business district.
Police initially believed the cause was an exploding gas cylinder, but experts and officials later said it was almost certainly a bomb attack.
President Gloria Arroyo said the city's emergency services were on high alert, and 2,000 extra personnel had been drafted in "to prevent a similar occurrence".
"We assure everyone that a full-blown investigation is now under way," she said on local TV.
Bomb theory
The blast caused extensive damage to the shopping centre. Concrete blocks and planks fell from the roof and cars outside the mall were covered with debris.
Philippine police are investigating the cause of the blast
"It left a deep crater at the foot of the escalator," said Charlie Nepomuceno, an employee at the Glorietta complex.
"It also ripped open the roof of the building. I saw a man thrown on to the roof who had lost a leg," he told Reuters.
National police chief Avelino Razon said he believed a bomb had caused the explosion.
"Beyond that we can't say anything else yet because we are still investigating," he told AFP news agency.
Militants have targeted the Philippine capital in the past.
In 2004, more than 100 people died when Islamic militants from the Abu Sayyaf group - who are battling the military in the south of the country - blew up a passenger ferry in the capital.
And in February 2005, four people died in a bomb attack on a Manila bus.
Una explosión en una tienda exclusiva en el principal distrito financiero, Makati, de la capital de Filipinas, Manila, dañó el Glorietta Mall y causó la muerte de al menos ocho personas e hirió a otras 126 personas; 98 de éstas últimas requirieron hospitalización mientras que las otras 28 fueron dadas primeros auxilios en heridas menores en el sitio de la explosión.

El siniestro ocurrió a la 1:30 p.m. (hora local) luego de que los empleados de las distintas compañías en Makati terminaron sus almuerzos. Autoridades policiales reportaron inicialmente que la causa de la explosión fue el filtrado de gas de un tanque gas licuado del petróleo, pero luego de unas horas el Jefe de la Policía Nacional de Filipinas, el Director General Avelino Razon Jr, descartó la posibilidad de que un tanque de GLP haya causado la explosión, declarando que la explosión podría haber sido causada por una bomba ya que los investigadores en el sitio sintieron olor a pólvora y no GLP.

La Presidente de Filipinas, Gloria Macapagal-Arroyo, se encontraba en camino al sitio de la explosión. Por el momento, la explosión sigue bajo investigación y el escuadrón de explosivos se encuentra en el comercio para verificar que no haya residuos de bombas o restos humanos.