Abner Gómez, head of the disaster management agency, said none of the customers of Puerto Rico's Electric Power Authority had any electricity.
The US National Hurricane Center said "catastrophic" flooding was sweeping parts of the island.
Puerto Rico Governor Ricardo Rossello said one fatality had been reported so far, a man struck by flying debris.
Maria, weakened to a category two storm, is moving away from the US territory and heading towards the Dominican Republic.
"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."
Those who emerged from their homes in San Juan after the storm's passing found streets strewn with debris, fallen power lines and shredded trees.
Mr Rossello issued a curfew for 18:00 local time (22:00 GMT) on Wednesday evening through early Saturday morning.