General Santos (Philippines) | Dozens of rescuers in hard hats scrambled out of a partially collapsed grocery in a southern Philippine city on Wednesday as it was rattled by an aftershock from a powerful earthquake that left at least 45 people dead and 17 others missing in the region.

A safety officer blew his whistle, and others screamed to warn about 30 firefighters and coast guard personnel to dash to safety as concrete debris crashed down from the leaning three-story building in General Santos City in a frantic scene witnessed by an AP video journalist.

The coastal city, a bustling commercial hub and the country's tuna capital, was devastated by a 7.8-magnitude earthquake that hit on Monday and left a trail of destruction across southern Mindanao, the Philippines' second-most populous region.

"It was a strong aftershock, and an alarm was immediately sounded so those inside and under the damaged building could run out for a headcount," said Ressa Mia Tactaquin-Betoya, who speaks for the firefighters searching for the last employee missing in the ruined grocery, where two upper floors collapsed during the initial quake.

"It was scary because we don't want our rescuers to be harmed, so the area must be secured before they can go back in," she told The Associated Press.

The earthquake has been followed by more than 2,100 aftershocks, including a few that ranged up to 6.4 magnitude, which is strong enough to cause more casualties and damage, according to Teresito Bacolcol, who heads the Philippines Institute of Volcanology and Seismology.

More than 25,000 people remain displaced, many of them staying in 45 government-run emergency shelters and still too traumatised to return home, officials said.

Monday's quake was one of the most powerful to hit the Philippines in half a century. It injured at least 630 people and damaged more than 3,100 houses, 29 roads, 11 bridges and more than 100 government buildings.

It also damaged the international airport in General Santos, forcing it to shut down indefinitely except for government and military flights transporting aid and disaster-response personnel, Civil Aviation Authority of the Philippines spokesperson Eric Apolonio said.

About 6,000 public school buildings in quake-hit provinces must be assessed before classes can resume. The quake struck on the first day of classes nationwide after a two-month summer break, and many who sustained injuries were young students who had gathered with excitement for morning flag-raising ceremonies.

Most of the deaths were caused by falling debris from collapsed buildings and landslides in General Santos and the nearby provinces of Sarangani, South Cotabato and Davao Occidental.

At least one person died after being swept out to sea following the quake, as waves up to 1.4 meters (4.6 feet) above tide level were measured in the southern Philippines. Smaller waves washed ashore in Indonesia and Palau and as far away as southern Japan before tsunami warnings were lifted.

Seven swimmers near General Santos were swept away by strong currents in the minutes after the quake. Three were rescued by the coast guard, one managed to swim back to shore, one drowned, and two remain missing, the Philippine coast guard said.

The strong currents that swept away the victims were most likely set off by the earthquake, Bacolcol said.

The earthquake was set off by movement in the Cotabato Trench and was one of the strongest to hit the country since the same undersea depression triggered an 8.1-magnitude quake that whipped up tsunami waves on August 17, 1976 and killed about 8,000 people.

The Philippines, one of the world's most disaster-prone countries, is often hit by earthquakes and volcanic eruptions due to its location on the Pacific "Ring of Fire," an arc of seismic faults around the ocean. (AP