The Pacific Tsunami Warning Center said there was a risk big waves could spread 300 km (190 miles) away from the epicenter, reaching as far away as Indonesia.
The Philippines Institute of Volcanology and Seismology (Philvolcs) said no tsunami warning was issued after a quake it had measured at 7.2. The U.S Geological Survey downgraded its own assessment of the magnitude to 6.8.
The Philippines is on the geologically active Pacific Ring of Fire and experiences frequent earthquakes, particularly in the south, although there were several last month in Batangas on the main island of Luzon, felt also in the capital Manila.
The earthquake was about 57 km (35 miles) southwest of Sarangani province, the southernmost part of Mindanao, the biggest island in the south of the archipelago nation.
"It was so strong, you could hear the ground shaking for almost 10 seconds," Jenifer Bugtay, who works at a beach-front resort in Sarangani, said by telephone.
"This is the strongest earthquake I have experienced," she said. "The wall looked like it would fall."
Philvolcs head Renato Solidum said there was no need to evacuate the area. The quake was at sea, and deep, although the public had been advised to stay away from the coast in the south for the next few hours.
Solidum said no major damage was anticipated.
"Many of our southern cities are far from the epicenter so we are not expecting much damage," he told news channel ANC.
"We have had some aftershocks, we expect that."
Janet Bongolan, tourism officer at the Tuka Marine Park in Sarangani province, said people spilled out of their homes and into the streets during the earthquake, but most had returned and there was no sign of panic.
"There's no news here that there will be a tsunami. But we are watching out for aftershocks. We are careful here," she said by telephone.