CBS News Correspondent David Begnaud said about 750,000 customers are also without water on the island.
Hundreds of people were evacuated or rescued across the island as floodwaters rose swiftly. Rushing rivers of brown water enveloped cars, first floors and even an airport runway in the island's southern region.
“The damages that we are seeing are catastrophic,” Gov. Pedro Pierluisi said.
“I urge people to stay in their homes,” said William Miranda Torres, mayor of the northern town of Caguas, where at least one large landslide was reported, with water rushing down a big slab of broken asphalt and into a gully.
U.S. President Joe Biden declared a state of emergency in the U.S. territory as the eye of the storm approached the island's southwest corner.
"As a result of the bad weather, including the 80 mph winds of Hurricane Fiona, the electrical system has suffered several interruptions in the transmission lines, which contributed to a blackout through the island," LUMA Energy said in a statement on Sunday.
"Due to the magnitude and extent of the blackout, as well as the effects of Hurricane Fiona, the total restoration of electricity service could take several days."
The storm's clouds covered the entire island and tropical-storm-force winds extended as far as 140 miles from Fiona's center.
Anxiety ran high across the island with Fiona due just two days before the anniversary of Hurricane Maria, a devastating Category 4 storm that hit on Sept. 20, 2017, destroying the island's power grid and causing nearly 3,000 deaths.
More than 3,000 homes still have only a blue tarp as a roof, and infrastructure remains weak.
The storm was forecast to pummel cities and towns along Puerto Rico’s southern coast that have not yet fully recovered from a string of strong earthquakes that hit the region starting in late 2019.
Puerto Rico’s power grid was razed by Hurricane Maria and remains frail, with reconstruction starting only recently. Outages are a daily occurrence.
No deaths had been reported, but authorities in the U.S. territory said it was too early to know the full scope of damage from an expansive storm that was still forecast to unleash torrential rain across Puerto Rico on Monday.