Tsunami warnings were issued on Monday in the Philippines, neighbouring Indonesia by the US Tsunami Warning System, with people in coastal areas warned to move to higher ground.
The Philippine Institute of Volcanology and Seismology (Phivolcs) put the magnitude at 7.8.
The Pacific Tsunami Warning Center initially put the magnitude at 8.2 and said the quake struck at a relatively shallow depth of 63km..
Phivolcs issued a tsunami warning, saying waves of up to three metres were possible.
Indonesia's geophysics agency BKMG said waves of 20cm had been detected so far.
DZBB radio, broadcasting from the Philippine city of General Santos about 15km from the epicentre, reported instances of falling furniture, damage to televisions and other appliances as the area experienced aftershocks and people left their homes to seek safety.
The General Santos disaster office said aftershocks were still being felt and authorities were assessing reports of damage and some injuries.
In the Philippines' Sarangani province near the epicentre, power and telecommunications were down and school classes were suspended, said local disaster chief Rene Punzalan, adding a damage assessment was underway with no reports yet of any collapsed buildings.
Philippine President Ferdinand Marcos Jr said authorities were moving fast to co-ordinate disaster response.
"The national government is moving and we will not leave Mindanao behind," Marcos said in a statement.
The Philippines and Indonesia experience hundreds of quakes each year and sit on tectonically complex parts of the "Pacific Ring of Fire", a seismically active belt stretching from South America to the Russian Far East.
Benjie Ancheta, police chief of Alabel town in Sarangani, said the police building had some cracks after the quake, which occurred during their flag-raising ceremony, where some people fainted.
"This is the strongest earthquake we've experienced," Ancheta told Reuters by phone.
The German Research Centre for Geosciences said the 7.8 magnitude quake struck at a depth of 10km. It had earlier reported a magnitude of 8.2.
A spokesperson for Indonesia disaster mitigation agency said there were no reports of damage so far.
Arlene Hollero, disaster chief of Maasim town in the Philippines' Sarangani province, said their evacuation was underway in coastal villages and there were no reported casualties so far.
Water receded shortly after the quake, but the seas were normal so far, she said, adding a bridge suffered some cracks and a shrine with a huge cross collapsed.
"It's devastating," Hollero told Reuters by phone.
with dpa
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