Rescue teams in the Philippines are racing to reach survivors after a powerful magnitude 7.8 earthquake struck the country’s south, killing at least 37 people and injuring 400.

In the hard-hit city of General Santos on the island of Mindanao, rescuers on Tuesday searched the rubble of a collapsed commercial building that housed a grocery store, in search of two people still believed to be trapped inside.

Two survivors had been pulled from the debris, but a third was found dead, officials said. Scanners have so far detected no signs of life from those remaining.

“It’s difficult to accept, as a mother, that my son is still trapped there,” said Dioslinda Deluvio, distraught as she waited outside the building for news of her son.

“My only call is to have him retrieved today so we can be at peace.”

The quake struck shortly before 7:40am local time on Monday (23:40 GMT on Sunday), about 20km (12 miles) off the coast of Sarangani province, with tremors felt as far as Manado on the Indonesian island of Sulawesi, some 420km (260 miles) away.

A series of powerful aftershocks followed over the next two hours, with the largest measuring magnitude 6.5, according to the United States Geological Survey. The quake triggered tsunami warnings across several countries.

General Santos, a coastal city of about 720,000 people, bore the brunt of the destruction. At least 13 people were killed there in collapsed buildings and by falling debris.

The city has been placed under a state of calamity, with several buildings collapsed, streets strewn with debris and power lines downed.

At least 18 others died in Sarangani province, mostly in a landslide that buried houses in the mountainside town of Glan. Further deaths were reported in South Cotabato, Davao Occidental and Balut Island.

Some areas of Sarangani remain accessible only by helicopter, and fears of aftershocks were slowing rescue efforts.

“There are still aftershocks, so the rescuers are very cautious in their approach. That’s a challenge,” said regional civil defence chief Rodrigo Sosmena.

In General Santos, residents recounted the moment the ground began to shake.

“It was the first time I experienced something that strong, that I really couldn’t stop myself from tearing up,” said Jojo Calma, 44, who was driving past a building when it collapsed. “I thought about my children and my niece. What if something had happened to them?”

East of General Santos, in the town of Malita, more than 100 students and a dozen teachers had gathered for a flag-raising ceremony at a school when the quake struck, on their first day back after a two-month summer break. “Their excitement on the first day of school turned to trauma,” said school principal Rosavel Cachuela.

Most students remained seated and still, preventing injuries, she added.

About 2,000 houses and 117 government buildings were damaged across several provinces. About 6,000 public school buildings must be assessed before classes can resume. The international airport in General Santos remained closed, forcing the cancellation of 63 domestic flights.

Al Jazeera’s Barnaby Lo, reporting from the capital, Manila, said it was a miracle that the death toll was not in the hundreds, given the extent of the damage.

“The focus right now is search and rescue, and search teams are racing against time. Of course, they’re hoping they’d still be able to pull people who are trapped underneath the rubble alive,” he said.

“Now, one of the biggest challenges is trying to reach mountainous areas that have been affected by the earthquake, because dozens of roads and bridges have also been damaged. The public works department says its workers are working overtime to remove those roadblocks, and at the same time, government engineers are already inspecting buildings for their structural integrity, but it is a Herculean task,” he added.

President Ferdinand Marcos Jr said emergency agencies had been activated. “The national government is moving and we will not leave Mindanao behind,” he said.

The quake is the most powerful to hit the Philippines in eight months, following a magnitude 6.9 tremor off Cebu last year that killed 79 people.