(CNN) -- Puerto Rico residents spent the night outside as aftershocks rocked the island following a magnitude 6.4 earthquake that knocked out power and water services in some areas.
The quake was the strongest and likely the most damaging of hundreds of temblors that have struck the island since December 28. It hit before dawn Tuesday, leaving a man dead and causing dozens of homes and structures to crumble.
It was centered off Puerto Rico's southern coast, 6 miles south of Indios. Gov. Wanda Vázquez Garced declared a state of emergency and activated the Puerto Rico National Guard as she pleaded with residents to remain calm and prepare for aftershocks.
Terrified of sleeping indoors as aftershocks continue, neighbors put mattresses in their front yards while others spent the night Tuesday under white tents and tarps.
Riko Gonzalez and his parents were asleep in their home in Yauco, near Indios, when the quake struck. They scurried out of the house as dishes tumbled to the kitchen floor, he said.
Hundreds of aftershocks have hit the area in the past few days.
"People are afraid to go to bed, to then be woken up to worse earthquakes than the day before," Gonzalez said.
Water and power still an issue
Much of Puerto Rico is still without power Wednesday as engineers work to restore it in phases. About 500,000 customers are back on the grid, the Electric Energy Authority said in a tweet.
A system-wide power outage was reported after the Costa Sur power plant near Guayanilla suffered severe damage Tuesday, Vázquez Garced said.
"We lost the largest plant in the entire system," said Jose Ortiz, executive director of the Electric Power Authority.
Power has been restored in most hospitals, and crews are working to fully restore it by the weekend, Ortiz said. A nursing home in Ponce was evacuated and dozens of people in wheelchairs were waiting outside.
Other towns affected by earthquakes are Guanica and Yauco.
Classes have not resumed
Classes won't resume across the island until crews inspect all schools and confirm buildings are safe for students, education officials announced.
The Agripina Seda School in Guanica suffered major damage Tuesday, including a partially collapsed, three-story building.
"Classes in the public school system won't resume until a total evaluation of all campuses," Education Secretary Eligio Hernández Pérez tweeted, adding that teachers and staff won't return to the schools until further notice.
Damage worse than hurricane, official says
The earthquakes come after Hurricane Maria devastated the US territory in September 2017. Many in southern Puerto Rico said the earthquakes' damage was worse.
"There's no warnings for this," Puerto Rico Police Commissioner Henry Escalera said of the earthquakes. "A hurricane gives us time to plan ahead."
When asked what concerns him the most about the quakes' aftermath, he said, "That homes will not be safe to live in and the possibility of a collapse that will cause a person's death or serious injuries."
The Federal Emergency Management Agency announced aid has been made available to supplement local response efforts.
President Donald Trump's action authorizes FEMA to coordinate all disaster relief efforts and provide appropriate assistance for required emergency measures to save lives and protect property.