An improvised explosive was thrown on Monday night on the roof of the gasoline station in Tacurong city on Mindanao island, which has been plagued by a decades-old Muslim insurgency.
Two soldiers, three policemen and two civilians were wounded when a second blast went off minutes later as security forces were about to cordon off the area, said regional police spokesman Superintendent Romeo Galgo.
Two suspects have been arrested but their identities and the motive for the attack were still under investigation, Galgo added.
"We cannot conclude yet that this is an act of terrorism. We are looking at all angles as there are threat groups in surrounding areas," Galgo told AFP, referring to Muslim rebels in the strife-torn province of Maguindanao.
Nearby towns are also home to communist rebels waging one of Asia's longest insurgencies and the incident could have been linked to extortion, Galgo added.
The blasts occurred in a section of Mindanao that had been troubled by the Muslim insurgency and high crime.
Mindanao is the ancestral homeland of the Muslim minority in the largely Catholic Philippines.
The Moro Islamic Liberation Front, the largest Muslim rebel organisation, is in peace talks with the government but other extremist gangs that have pledged allegiance to the Islamic State group routinely carry out bombings and kidnappings.