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.
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 leader Fidel Castro said Tuesday in a statement that his health was stable after undergoing intestinal surgery and that he was in good spirits.
A statement by Castro read on state television began with Castro saying: "I cannot make up positive news."
But he said his health was "stable," and "as for my spirits, I feel perfectly fine."
Castro also expressed his gratitude for all the good wishes he has received from government leaders and supporters around the world, and called on Cubans to remain calm and maintain their daily routines.
Castro apologized for not giving more details but said the threat posed to his government by the United States means his health must be treated as "a state secret."
The Cuban president announced Monday he would temporarily hand power to his brother Raul and "undertake several weeks of rest" following his operation.
Cuba's speaker of parliament, Ricardo Alarcon, told the government's Prensa Latina news service earlier Tuesday that the Cuban leader is known for fighting to the very end, but said his "final moment is still very far away."
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 the statement read 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 in 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."