Castro died in Havana aged 90, his brother, President Raul Castro, announced on national television.
The BBC's Havanna correspondent Will Grant said people are still processing the news about his death.
Some young Cubans are seated under one of the few wifi-spot in the country soaking in the news, he said.
"The commander in chief of the Cuban revolution died at 22:29 hours this evening," the president announced on national television.
Raul Castro, who took power after his older brother Fidel was hospitalized in 2006, said that the revolutionary leader's remains will be cremated early on Saturday, "in compliance with his expressed will."
One of the leading world figures in the second half of the 20th century, Fidel Castro had outsized influence given the size of his small Caribbean island.
He was said to have survived countless US assassination attempts.