Abner Gómez, head of the disaster management agency, said the hurricane had damaged "everything in its path".
None of the customers of Puerto Rico's Electric Power Authority had any electricity, he said.
Maria weakened to a category two storm with winds of 110 mph (177km/h) as it moved away from Puerto Rico.
"When we are able to go outside, we are going to find our island destroyed," Mr Gómez was quoted as saying by the El Nuevo Dia newspaper.
"It's a system that has destroyed everything in its path."
The US territory's governor, Ricardo Rossello, issued a curfew for 18:00 local time (22:00 GMT) on Wednesday evening through early Saturday morning.
Earlier, Mr Rossello asked President Donald Trump to declare the island a disaster area after the storm unleashed heavy flooding and life-threatening winds.
He said major damage was inevitable, although 500 shelters had been established to protect people.
The devastating storm has already left seven people dead on the island of Dominica, which was badly affected on Monday.
Aerial footage shows flattened houses and the death toll on Dominica is likely to rise, with details remaining scant as communication links are down.
Photo by: REUTERS (Caption: Rescue workers help people in Guayama, Puerto Rico)