The quake struck off Mindanao island at a depth of 60 km (37.3 miles), according to the German Research Center for Geosciences (GFZ).
PHIVOLCS director Teresito Bacolcol told DZRH radio the earthquake's intensity was "destructive, so we would expect damage".
The earthquake lasted several seconds, he said, advising residents to be on alert for aftershocks as strong as magnitude 6.2.
Radio announcer Leny Aranego in General Santos city, close to Glan, said the quake shook buildings and desks.
Michael Ricafort, an economist at the lender RCBC, said in a Facebook post passengers at General Santos City airport were evacuated to the tarmac.
Persons:
Angel Dugaduga, Amor Mio, Mio, Teresito Bacolcol, Leny Aranego, Michael Ricafort, Shubhendu Deshmukh, Neil Jerome Morales, Mikhail Flores, Kanupriya Kapoor, Andrew Cawthorne
Organizations:
Fire, German Research Center, Geosciences, General Santos, Thomson
Locations:
Philippines, Fire MANILA, Mindanao, Glan, Earthquakes, Philippine, Koronadal, General Santos, Bengaluru, Manila