Puerto Rico Governor Wanda Vazquez declared a state of emergency and activated the National Guard on Tuesday, El Nuevo Dia newspaper reported, after a series of earthquakes including one of magnitude 6.4 struck the Caribbean island.
The temblors killed at least one person, knocked out power across much of the island and caused significant damage, authorities and media reported.
Vazquez said all public sector offices except for emergency services would remain closed on Tuesday while emergency plans were implemented.
The island has been rocked by a series of quakes in recent days, including a 5.8-magnitude temblor on Monday that damaged a few homes on the southern coast.
The U.S. territory is still recovering from a pair of devastating 2017 hurricanes that killed about 3,000 people and destroyed significant infrastructure across an island working through a bankruptcy process to restructure about $120 billion of debt and pension obligations.
Vazquez, who assumed office in August after Ricardo Rossello stepped down in the face of massive street protests, tweeted pleas for people to remain calm.
"We want everyone to be safe. That is why all work in the public sectors has been suspended today, so that you can be with your family, implementing your emergency plans," Vazquez tweeted.
The Pacific Tsunami Warning Center reported a small tsunami measuring around 20 centimeters.
The first and biggest quake on Tuesday, of magnitude 6.4, struck at a depth of 10.0 km at 4:24 am (0824 GMT) near Ponce on the island's southern coast, the US Geological Survey said.
A 73-year-old died there after a wall fell on him, newspaper El Nuevo Dia reported.
Witnesses on social media described the quake as "super strong" and lasting up to 30 seconds. It was followed by a number of hefty aftershocks including one measuring 5.8.
The impact along the country's southern coast appeared significant.
Puerto Rico Electric Power Authority Chief Executive Jose Ortiz said its Central Costa Sur power plant was damaged, and the utility was checking other substations on the island.
PREPA had cut off power on safety grounds and hoped to reconnect supplies as soon as possible, it said on Twitter.
The quake severely damaged the Immaculate Conception church in Guayanilla, leaving about half of it standing and surrounded by piles of rubble, according to video posted by Wapa TV. A picture published by El Nuevo Dia showed people removing artifacts.
At least eight homes collapsed in Yauco, El Nuevo Dia reported, citing Mayor Angel Torres. Wapa TV video showed one home in Yauco flattened, its roof intact atop debris and slanting until it touched the ground.
The international airport in Carolina, just east of San Juan, continued normal service with the help of power generators, El Nuevo Dia reported, citing Jorge Hernandez, chief executive of Aerostar Airport Holdings.
In the town of Guanica, several buildings collapsed. Further east in Maunabo, video on social media showed people evacuating to higher ground following the tsunami warning.
"Persons along coastal areas near the earthquake should be observant and exercise normal caution, otherwise no action is required," the PTWC said in its warning.
Monday's quake off southern Puerto Rico knocked several houses off their supporting pillars in Guanica and Guayanilla, crushing vehicles beneath them.
That quake also destroyed the Window of the Caribbean, a rock formation on a beach that had been a tourist attraction, but there were no reports of injuries.