A strong magnitude 6.7 earthquake shook the southern Philippines on Friday, killing a married couple as people fled buildings and part of a shopping mall ceiling collapsed.

No tsunami threat was issued after the quake struck off Sarangani province on the main southern island of Mindanao at a depth of 78 kilometers (48 miles) at 4.14 pm (0814 GMT), the US Geological Service said in a statement.

A man and his wife were found crushed to death beneath a concrete wall outside a wood factory near General Santos City where the couple worked, city police said.

"The wall fell on them" during the quake, Corporal Christopher Larano told AFP.

The quake was felt across a wide area of the mountainous island.

"I think it was the strongest earthquake I've ever experienced," Keeshia Leyran, 27, told AFP from Davao City, about 200 kilometers (124 miles) from the epicenter, where she was attending a conference.

"People around me were panicking and running to go outside."

A photo shared on Facebook and verified by AFP showed a collapsed ceiling inside a shopping mall in General Santos, less than 100 kilometers from the epicenter.

A video showed terrified shoppers in another mall in the same city hiding under fast-food restaurant tables and screaming as the building shook and pieces of the ceiling fell.

"We were on the second floor so there was nothing we could do but duck under the tables," said Gregorio Narajos, 34, who shot the video.

"Power went out two or three seconds into the quake. It came back after a while," Narajos said. 

"I saw stuff falling from the ceiling. When we got out we saw cracks and debris outside."

About 30 students at a nearby high school were treated for breathing difficulties due to panic following the quake, said Adrian Imbong, an emergency medical services worker in General Santos.

Sarangani municipality police officer Captain Giecarrjune Villarin said he and his colleagues fled their building, which is about 30 kilometers southeast of where the quake struck.

"We saw people run out of a nearby gym where they had been playing basketball," he said. 

"We have not received any reports of damage or casualties."

No 'substantial damage'

Quakes are a daily occurrence in the Philippines, which sits along the Pacific "Ring of Fire", an arc of intense seismic as well as volcanic activity that stretches from Japan through Southeast Asia and across the Pacific basin.

Most are too weak to be felt by humans, but strong and destructive ones come at random with no technology available to predict when and where they will happen.

Raquel Balaba, 58, was with her grandchildren at a primary school in General Santos when they felt the ground move.

"We were in an open field but the earthquake was so strong that the children got really scared and started crying," Balaba told AFP.

Some schools in Jose Abad Santos municipality in Davao Occidental province reported cracks in their buildings, said Jason Sioco, a member of the local disaster agency.

But he said there had been no reports of injuries or "substantial damage".

"Power and telephone signals were cut off for a while but electricity is back on," he said.