Long-time Cuban leader Fidel Castro is temporarily handing over power to his brother as he has surgery, his spokesman Carlos Valenciaga told the nation in a TV statement.
As the news spread, Cuban expatriates took to the streets of Little Havana, a traditional home of Cuban exiles in Miami, Florida, to celebrate.
Many mocked Castro, the man they accuse of being a ruthless dictator. Some speculated the 79-year-old had died.
Others proclaimed they were the true Cuban patriots.
The party went on into the early hours.
The reaction was more muted in Cuba, although correspondents said the news would come as a shock to the majority, who have known only Castro as their leader.
Just days ago, Castro addressed thousands of flag-waving Cubans on the anniversary of the 1953 Moncada barracks assault which sparked the Cuban revolution.