Cuban leader Fidel Castro said Monday he is undergoing surgery and will temporarily transfer his presidential powers to his brother, Raul.
Castro said in a letter read live on television by his secretary that he has suffered gastrointestinal bleeding, brought on by stress from recent public appearances in Argentina and Cuba.
"This caused an acute intestinal crisis with sustained bleeding," said the statement read out by aide Carlos Valenciaga.
Castro, who turns 80 on Aug. 13, has been leader of the country since toppling the regime of Fulgencio Batista in 1959.
News of Castro's surgery sparked celebrations in Miami's Little Havana overnight. Cuban exiles living in the city waved flags and took to the streets shouting "Cuba! Cuba! Cuba!"
Castro did not appear in public to announce the switchover, and a planned birthday celebration has been postponed until December, on the 50th anniversary of Cuba's Revolutionary Armed Forces.
He has been the world's longest-ruling head of government. Only Britain's Queen Elizabeth, crowned in 1952, has been head of state longer.
Castro shattered a kneecap and broke an arm when he fell after a speech on Oct. 20, 2004, but has consistently brushed off any concerns about his failing health.
Raul Castro, 75, has been second-in-command since that time. He will take over as first secretary of the ruling Communist Party, commander in chief of the armed forces and president of the executive council of state
Cuban President Fidel Castro remained in hospital Tuesday after undergoing intestinal surgery, with the government issuing no new statements on the health crisis that has forced him to temporarily hand power to his brother Raul.
World leaders also said they were closely monitoring the health of the leader of the Caribbean communist country, whose announcement Monday that he would "undertake several weeks of rest" following his operation stunned Cubans both on the island and in exile.
Castro has wielded absolute power in Cuba for 47 years, while his 75-year-old brother heads the country's armed forces.
Castro underwent intestinal surgery to stem "sustained bleeding," his personal secretary said in a statement on Cuban state media on Monday.
The statement, signed by Castro, blamed his illness on the stress of his recent trip to Argentina and last week's anniversary of the start of the armed Cuban revolution.
"Days and nights of continuous work with hardly any sleep have caused my health, which has withstood all tests, to fall victim to extreme stress and to be ruined," the statement said.
"Imperialism will never be able to crush Cuba," another section of the statement said. "The Battle of Ideas will continue."
Raul Castro was officially designated in 1997 as the president's successor in case of illness or death.
Celebrations break out in Little Havana
As news of Fidel Castro's illness broke, less than two weeks before his 80th birthday, Cuban expatriates celebrated in the streets of Little Havana in Miami, causing traffic gridlock in that section of the Florida city.
There were no reports of arrests as the cheering, dancing in the streets and flag-waving continued overnight Monday and into Tuesday.
Over nearly five decades, hundreds of thousands of Cubans have fled Castro's rule, many of them settling in Miami and other southern Florida communities.
World's eyes are on Castro's health
On Tuesday, the administration of U.S. President George W. Bush said it was monitoring the health crisis, and officials restated their goal of helping to ensure a democratic transition on the island.
Spain's Socialist government wished the Cuban leader a "speedy recovery."
"We are closely following the situation as we would for any world leader," said a spokesperson for Spain's Foreign Affairs Ministry.
The U.S. and Cuba have been political adversaries since Castro entered into an alliance with the Soviet Union and converted his country into a Marxist-Leninist state in the early 1960s.
Venezuelan President Hugo Chavez, Castro's strongest international ally, called Castro's office after hearing the news during a visit to Vietnam.
"Waking up this morning and receiving that news, you may see what feeling one would have toward a good friend," Chavez said Tuesday morning. "When there is such an announcement, it's worrisome.
"We wish President Fidel Castro will recover rapidly," Chavez said. "Viva Fidel Castro."
Cuba's Communist government tried to impose a sense of normalcy Tuesday, its first day in 47 years without Fidel Castro in charge. A senior Cuban official insisted Castro's final moment was "very far away," despite his handing over power to his brother after surgery.
Parliament Speaker Ricardo Alarcon said the Cuban leader is known for fighting to the very end and that his "final moment is still very far away," the government's Prensa Latina news service reported.
Raul Castro, the island's acting president, was nowhere to be seen as Cubans began to worry about what comes next and exiles in Miami celebrated a development they hoped signaled the death of a dictator. Cuban dissidents kept a low profile while watching for signs of Castro's condition.
"Everything's normal here - for the moment," said hospital worker Emilio Garcia, 41, waiting for a friend at a Havana hotel. "But we've never experienced this before - it's like a small test of how things could be without Fidel."
The main newscast on state-run TV gave no details of the 79-year-old leader's condition, but ran a string of man-on-the-street interviews with Cubans wishing him well and professing confidence in the revolution's staying power. The anchor said Castro had the people's "unconditional support."
It was unknown when or where the surgery took place or where Castro was recovering. But the Venezuelan government, Cuba's closest ally, said Cuban officials reported Castro was "advancing positively." Leftist Argentine lawmaker Miguel Bonasso said Castro aides told him the leader was resting peacefully.
Cubans were stunned when Castro's secretary read a letter on state television Monday night announcing their leader was temporarily turning over power to his younger brother, the island's defense minister and the president's designated successor.
In the letter, Castro, who turns 80 on Aug. 13, said doctors operated to repair a "sharp intestinal crisis with sustained bleeding." Neither Castro brother was shown.
Castro had been seen frequently in recent days, delivering speeches in eastern Cuba during a revolutionary holiday and making waves at a trade summit in Argentina. Those back-to-back trips and the resulting stress "ruined" his health, according to his letter.
"It's so surprising, because in Argentina he gave off such a strong political image and looked quite vital," said Rafael Marti, a businessman from Spain visiting Cuba with his wife. He said he didn't expect rapid change on the island 90 miles south of Florida.
Cubans agreed nothing was likely to change overnight - especially not with Castro's fiercely loyal brother at the helm. Raul Castro, who turned 75 in June, has been his brother's constitutional successor for decades and has assumed a more public profile in recent weeks.
The calm delivery of the announcement appeared intended to signal that any transition of power would be orderly. Yet some feared resentment over class divisions could spark conflict if a political vacuum develops.
"It's better for things to move slowly, instead of abrupt change," Garcia said. "But people are a bit nervous - anything could happen."
Dissidents said they expected the government to be on the defensive, with a high security presence and a low tolerance for political acts.
"It's clear that this is the start of the transition," said activist Manuel Cuesta Morua. "This gives Cuba the opportunity to have a more rational leadership" because top leaders will be forced to work together rather than following one man.
Officials halted some interviews by journalists Tuesday, with one plainclothes officer ejecting an Associated Press reporter from a cafe for asking questions. People on the street were reluctant to talk to foreign journalists, and many declined to give full names.
"We've been asked to keep things normal here, and to make sure that the revolution continues," said Daniel, a young social worker.
Government work centers brought employees together for small rallies throughout Havana.
"For this man, we must give our life," a customs worker told a crowd waving Cuban flags and shouting "Long live Fidel!"
Elsewhere, it looked like a regular day in Havana, with people packed into buses and standing in line outside stores.
Across the Florida straits in Miami, where hundreds of thousands of fleeing Cubans have settled, boisterous celebrations Monday night gave way to speculation about what would happen in Cuba when Castro dies. Car horns still blared, but some cautioned the celebrations may have been premature.
Many Cubans on the island thought the Miami celebrations were in poor taste.
"We aren't going to celebrate someone's illness," said a waitress who wouldn't give her name.
In Washington, the State Department said it would support a democratic transition in Cuba. Spokesman Sean McCormack said the Cuban people are weary of communist rule and eager to choose a new form of government.
"We believe that the Cuban people aspire and thirst for democracy and that given the choice they would choose a democratic government," he said.
Castro, who took control of Cuba in 1959, has resisted repeated U.S. attempts to oust him as well as demands for multiparty elections and an open economy. He has survived communism's demise elsewhere and repeatedly insisted his socialist system would long outlive him.
Doctors in the United States said Castro's condition could be life-threatening but since the details of his symptoms were not released it was hard to say what caused the bleeding: severe ulcers, a colon condition called diverticulosis or - an outside possibility - cancer.
Castro seemed optimistic of recovery, asking in his letter that celebrations scheduled for his 80th birthday be postponed until Dec. 2, the 50th anniversary of Cuba's Revolutionary Armed Forces.
The leaders of China, Venezuela, Bolivia and Mexico wished Castro well.
Castro has been in power since the Jan. 1, 1959, triumph of the armed revolution that drove out dictator Fulgencio Batista. He has been the world's longest-ruling head of government, and his ironclad rule has ensured Cuba's place among the world's five remaining communist countries, along with China, Vietnam, Laos and North Korea.
Talk of Castro's mortality was taboo until June 23, 2001, when he fainted during a speech in the sun. Although Castro quickly recovered, many Cubans understood for the first time that their leader would eventually die.
Castro shattered a kneecap and broke an arm when he fell after a speech on Oct. 20, 2004, but laughed off rumors about his health, most recently a 2005 report he had Parkinson's disease.
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But analysts say the younger Castro, the world's longest serving defense minister, is no obsequious subordinate.
Rather, he appears to be a savvy organizer who has helped keep the leftist firebrand Fidel Castro in power with the backing of an efficient military, and now finds himself flung into a perhaps unwelcome public limelight after his brother underwent an intestinal operation.
Raul Castro, once a communist hard-liner, is seen as more pragmatic now than his brother when it comes to plotting the future of Cuban communism, and could lead Cuba to follow China's model of one-party politics and free market economics.
"Beans are more important that cannons," Raul said in the early 1990s, when Cuba was plunged into severe economic crisis after the collapse of the Soviet Union and the military was forced to park jet fighters with the help of horses for lack of fuel.
The 50,000-strong Revolutionary Armed Forces he runs were the first institution on the island to adopt capitalist business practices, importing British manuals on management, and it now has major stakes in industry, agriculture and tourism, owning hotels at beach resorts and an airline.
His public profile has grown in recent years when he spearheaded a drive to reinvigorate the Communist Party at grass-roots level, and to heighten Cuba's military alertness since President Bush took power.
Cuba watchers have, however, long said Raul lacked the charisma, political skill, the health or even the ambition to completely fill his brother's shoes.
Some see him serving as a brief transitional figure who would open up Cuba, a mediator for a "softer" communism or a figurehead for a younger collective leadership.
Ricardo Gonzalez, a former president of the Miami-based Cuban Committee for Democracy, said socialism and one-party rule are probably too deeply ingrained in Cuba to be wiped out by the eventual death of Fidel, the "Comandante."
"I think it's going to be more like when Stalin died, when Mao died. It will continue, It's too institutionalized, and there's nothing else that can take its place," said Gonzalez.
Many exile leaders are betting that Fidel Castro's eventual departure will prompt a speedy transition.
"There's a lot of talk of this succession plan where Raul Castro is being incorrectly portrayed as the kinder, gentler dictator," said Ileana Ros-Lehtinen, a Cuba-born Republican congresswoman from South Florida and strident Castro critic.
"I think that were Raul to grab onto power it would be short-lived," Ros-Lehtinen told Reuters before the news that Fidel Castro had relinquished power.
Raul's chief of staff for 10 years, Alcibiades Hidalgo, who defected on a boat to the United States in 2002, said Raul always drank too much and was not in good health.
But since the failed assault on the Moncada Barracks, the guerrilla war in the Sierra Maestra hills and the triumph of the January 1, 1959 revolution, Raul has always been his brother's most trusted right-hand man.
Military comrades describe him as tough but fair.
With Soviet aid, he helped convert a ragtag guerrilla force into one of the most formidable armies of the Third World. Cuban military advisors and troops served in "anti-imperialist" conflicts around the world, including Vietnam, Ethiopia and Angola.
Born on June 3, 1931, Raul Castro was raised -- like Fidel Castro -- on their father's large farm in eastern Cuba. As a boy, his best friends were the sons of poor peasants working for his well-to-do father, a fact which, he says, gave him a life-long sympathy for the underdog.
He is married to Vilma Espin, who took part in the revolutionary struggle, has headed the Cuban Federation of Women since it was founded almost forty years ago, and operates as Cuba's unofficial First Lady.