Fidel Castro, who took control of Cuba in 1959 and has rebuffed American attempts to oust him ever since, temporarily handed over power to his brother Raúl last night following intestinal surgery.

The Cuban President, less than two weeks away from his 80th birthday, did not appear on the live television broadcast in which his secretary read a letter announcing the transfer of authority.

In the note, read by Carlos Valenciaga, Señor Castro said he had suffered gastrointestinal bleeding, due to stress from recent public appearances in Argentina and eastern Cuba.

"The operation obligates me to undertake several weeks of rest," the letter read. Extreme stress "had provoked in me a sharp intestinal crisis with sustained bleeding that obligated me to undergo a complicated surgical procedure".

Señor Castro said he was temporarily relinquishing the presidency to Raúl, the Defence Minister who is his nominated successor, but said the move was of "a provisional character". There was no immediate appearance or statement by Raúl, who is 75.

The Cuban President asked that celebrations scheduled for his 80th birthday on August 13 should be postponed until December 2, the 50th anniversary of the formation of the country's Revolutionary Armed Forces.

In power since the triumph of the Cuban revolution on January 1, 1959, Señor Castro is the world’s longest-ruling head of government. Only King Bhumibol Adulyadej of Thailand, crowned in 1946, and Queen Elizabeth II, who acceded the throne in 1952, have been heads of state longer.

The "maximum leader’s" rule has ensured Cuba remains among the world’s five remaining communist countries. The others are all in Asia: China, Vietnam, Laos and North Korea.

Streets in Havana, including the coastal Malecon highway where young people often congregate, were quiet last night. In Old Havana, waiters at a popular cafe were momentarily stunned as they watched the news, but quickly got back to work. "He’ll get better, without a doubt," said Agustin Lopez, 40.

A leading Cuban government opponent in Havana said she believed Señor Castro must be gravely ill to have stepped aside temporarily. "It’s almost the same as death," Martha Beatriz Roque said in a telephone interview.

In Washington, a White House spokesman, Peter Watkins, said: "We are monitoring the situation. We can’t speculate on Castro’s health, but we continue to work for the day of Cuba’s freedom."

Hugo Chávez, the leftist President of Venezuela who is one of Señor Castro's leading supporters and a key source of subsidy for the Cuban economy, heard the news at the end of a world tour in Vietnam.

"With my heart, I wish that President Fidel Castro will quickly recover to always stay with us," he told a meeting of Vietnamese businessmen in Hanoi. He then raised his fist and shouted: "Long live Fidel Castro!"

Across the Florida Straits in Miami, anti-Castro exiles waved Cuban flags on Little Havana’s Calle Ocho, and shouted "Cuba! Cuba! Cuba!" There were hugs, cheers and dancing as drivers honked their horns. Over nearly five decades, hundreds of thousands of Cubans have fled Señor Castro’s rule, many of them settling in Miami.

Talk of Señor Castro’s mortality was taboo in Cuba for many years, with the President famous for his good health and ability to deliver staggering, eight-hour speeches.

But that ended on June 23, 2001, when he fainted during a speech in the sun. He then shattered a kneecap and broke his right arm when he fell after a speech on October 20, 2004.

In 2005 Señor Castro rejected rumours of ill health and that he had Parkinson’s disease, saying he felt "better than ever" and pointing out that Pope John II had managed to continue working despite the disease.

Although few Cubans speak openly of the succession plans for the Castro Government, Raúl, who has supported his brother since their early days of student opposition, has long been considered his natural deputy.

But his own advanced age — Raúl is just four years younger than Fidel — has led to speculation that a triumvirate of Communist Party officials, made up of Felipe Peréz Roque, the Foreign Minister, Ricardo Alarcón, the 69-year-old President of Cuba's National Assembly, and Carlos Lage, the Vice President who manages the Cuban economy, will form a newly constituted government.

Raúl Castro hinted at a collective government during a speech in June in which he said: "Only the Communist Party... can be the worthy heir of the trust Cubans have placed in their leader."

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