GUAYANILLA, PUERTO RICO - Puerto Rico's governor declared a state of emergency on Tuesday after a powerful 6.4 magnitude earthquake killed at least one person in the south of the island and caused widespread damage.

Governor Wanda Vazquez said the declaration would allow for the activation of National Guard troops in the US territory still recovering from a devastating 2017 hurricane.

The US Geological Survey said the quake struck at 4:24 am (0824 GMT) with the epicenter off the coast of the southern city of Ponce, and was followed by more than a dozen aftershocks.

Tuesday's quake was the most powerful in a series of tremors that have shaken the island since December 28.

Scientists initially sent out an alert about a potential tsunami but it was later canceled.

The island's electricity authority said the quake had forced an automatic shutdown of the power grid, already severely damaged by Hurricane Maria more than two years ago.

The worst damage appeared to be in towns on the southwest coast, including Ponce, Guayanilla and Guanica.

El Nuevo Dia newspaper said a 73-year-old man died after a wall fell in his home in Ponce. Eight others there were reported injured.

Two power plants in Guayanilla sustained major damage, the Puerto Rico Electric Power Authority said. The city could be without power for two weeks, its mayor Nelson Torres Yordan said.

Celebrity chef Jose Andres announced that a charity he runs, World Central Kitchen, had started serving meals and distributing solar-powered lamps in quake-hit areas.

Vazquez announced that $130 million in emergency aid funding will be disbursed.

On social media, people wrote of being shaken awake by the force of the quake.

One woman on Twitter said she had been "wrenched from sleep."

"Everybody is awake & scared all over," she posted.

In Guayanilla, the Inmaculada Concepcion church, built in 1841, was heavily damaged.

Volunteers salvaged statues and other valuable items from the ruins as a priest consoled distraught parishioners. -AFP