Police on Tuesday ruled out sabotage or terrorism.
The explosions late on Monday initially raised concerns of foul play in a capital jittery with rumours of coup plots linked to the months-long political crisis surrounding President Gloria Macapagal Arroyo, accused of rigging last year's presidential election.
She denies the allegations.
The blasts at Camp Bagong Diwa obliterated an ammunition bunker, damaged the nearby rehabilitation clinic for drug addicts and a gas station, toppled power poles and left a crater at least four metres deep. About 30 parked vehicles were tossed several metres.
The injured included 101 patients at the drug rehabilitation clinic, four elite police officers and two civilians, police chief Ameto Tolentino said.
Demonstrators this month urged
the president to resign
Metropolitan Manila police chief Vidal Querol said the explosions shattered windows at his office about 100 metres away.
He said the explosions could have been triggered by a lightning strike during a thunderstorm late Monday.
Interior Secretary Angelo Reyes sought to reassure the public overnight that the incident was an accident.
"There is nothing to be worried about. It was an accident, that's our initial findings, so we can go back to sleep," he said.
President out of town
The blasts occurred hours after Arroyo left for New York to attend UN meetings and the military was placed on full alert in the capital.
Police explosives and ordnance disposal chief Warlito Tubon said the destroyed munitions bunker had contained thirty 50kg bags of ammonium nitrate, 420 mines, C-4 explosives and anti-tank ammunition.
He ruled out terrorism or sabotage based on accounts of witnesses who said a series of small and large explosions followed a blackout during a thunderstorm.
Members of the rebel Abu Sayyaf
group are held at the camp
Police pointed to a blackened lightning rod they said showed that lightning struck the armoury.
The successive blasts prevented police and firefighters from immediately approaching to put out the fire.
The police head of the drug clinic, Bonaparte Francisco, said about 400 of the 2200 patients at the facility fled in panic, many of them shirtless, but that most later returned.
Fifty-four were unaccounted for by midday Tuesday.
SWAT teams immediately tightened security at the camp, where suspected Abu Sayyaf Muslim fighters are detained in a maximum-security jail.
Reyes said an investigation was under way to determine who would be held accountable for the accident.
In February 2004, fire engulfed a warehouse containing unexploded World War II bombs at the national police headquarters, also in the capital, triggering numerous explosions that wounded at least three firefighters and a police officer.