The powerful earthquake struck early on Monday, damaging buildings and prompting tsunami warnings across parts of the region, according to the Philippine Institute of Volcanology and Seismology (Phivolcs).
The tremor's epicentre was located south-west of Maasim town in Sarangani province, in the southern region of Mindanao, Phivolcs said.
The quake was strongly felt in General Santos City, where airport operations were suspended.
People in coastal areas were urged to move to safety immediately.
The quake was also felt in parts of Indonesia, particularly on the island of Sulawesi, where a tsunami warning was also issued. Japan likewise warned parts of its Pacific coastline of possible waves.
Reports of major damage to schools, hospitals, shopping malls and churches were also being checked, the agency said.
Electricity and telecommunications services were disrupted in affected areas, the disaster agency said.
School classes were cancelled, while work in government and private offices was also suspended.
The earthquake struck on the first day of the school year, during flag-raising ceremonies at schools and government offices, meaning many students, teachers and staff were already outside buildings.
Elene Marie Jane Gamboa, principal of Matanao National High School in Davao del Sur province, said one of the school buildings collapsed.
"Thank God we were having our flag-raising ceremony and everyone was outside," she told Manila radio station DZMM.
The US Geological Survey (USGS) initially put the magnitude at 8.2, but later also revised it down to 7.8.
According to the USGS, the quake struck at a relatively shallow depth of 55km and waves of up to three metres could occur along some coastal areas.
Philippine authorities said, however, that no unusually high waves had initially been observed and that the situation would continue to be monitored.
Philippine President Ferdinand Marcos Jr immediately ordered relief and protective measures and called on the population to follow the instructions of the authorities.
"Move to higher ground now. Do not wait," Marcos said in a statement.
He said that people's lives were more important than anything they might have to leave behind.
The president stressed that the government had instructed all relevant authorities to act immediately. He ordered the suspension of all classes in the affected areas of Mindanao.
"The safety of our children is the top priority," he said.
The region lies on the Pacific Ring of Fire, a belt of intense tectonic activity encircling the Pacific Ocean, where earthquakes and volcanic eruptions occur with particular frequency.
with dpa
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