Tourists take pictures of the collapsed wall of the ruins of an iconic landmark lighthouse in Guanica.

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.

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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.

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

The quake, just off the US territory's southern Caribbean coastline, was felt throughout much of the island.

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

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.

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.

Tourists walk torwards the collapsed wall of the ruins of an iconic landmark lighthouse in Guanica.

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.

'Be safe'

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

A 5.8 magnitude quake on Monday toppled some structures, caused power outages and small landslides, but did not result in any casualties.

It also destroyed a popular tourist landmark, Punta Ventana, a natural stone arch that crumbled on the island's southern coast.

Vazquez, the governor, said government employees were being given the day off on Tuesday to take care of their families.

Popular tourist landmark Punta Ventana, is destroyed after a 5.8-magnitude earthquake in Guayanilla.

"We want everyone to be safe," she said.

"Obviously we all have all of our assets out in the street in the areas with major damage," Vazquez added.

She said ports were undamaged and there are several weeks of gasoline, diesel and natural gas stored so people need not worry about shortages.

The White House said President Donald Trump had been briefed and Pete Gaynor, head of the Federal Emergency Management Agency (FEMA), had been in touch with the governor.

Trump has been accused by some Puerto Ricans of failing to provide the island with adequate assistance following Hurricane Maria in 2017.

The Category 4 storm destroyed the island's already shaky power grid, overwhelmed public services, left many residents homeless and claimed several thousand lives, according to government estimates.

Agence France-Presse