He told Al Jazeera that police and military experts were looking into what kind of bomb was used to give an indication of what group could be behind it.
"We have fielded more than 2,000 police officers in the entire national capital region to increase the security measures," he said.
Speaking from the site after the blast, Al Jazeera's correspondent Marga Ortigas said: "The shopping mall lights are completely shut. The people are not being allowed in.
"Very few rescue workers are now working as they are afraid the foundation of the bomb site might collapse.
"They are worried there are still people buried under piles of rubble."
A general alert was issued for the rest of the city and for the international airport, officials said.
Jonjon Binay, a city councillor, said four people died immediately and four more died in hospital.
Blocks of cement had fallen from an upper storey of the shopping centre, hitting cars parked below and spreading a film of dust. Scores of windows in nearby shops were shattered.
A security guard said: "I was eating lunch when the ground shook. I thought it was an earthquake. Then the electricity went off."
Manila has largely been spared a spate of bomb attacks by
Muslim separatists have been fighting in the southern Mindanao region, but few attacks have been carried out in the capital. A series of bomb blasts in 2000 killed at least 22 people.