HAVANA - Fidel Castro temporarily relinquished his presidential powers to his brother Raul on Monday night and told Cubans he underwent surgery. The Cuban leader said he had suffered gastrointestinal bleeding, apparently due to stress from recent public appearances in Argentina and Cuba, according to the letter read live on television by his secretary, Carlos Valenciaga. "The operation obligates me to undertake several weeks of rest," the letter read, adding that extreme stress "had provoked in me a sharp intestinal crisis with sustained bleeding that obligated me to undergo a complicated surgical procedure." Castro said he was temporarily relinquishing the presidency to his younger brother and successor Raul, the defense minister, but said the move was of "a provisional character." There was no immediate appearance or statement by Raul Castro. The elder Castro asked that celebrations scheduled for his 80th birthday on Aug. 13 be postponed until Dec. 2, the 50th anniversary of Cuba's Revolutionary Armed Forces. Castro said he would also temporarily relinquish his duties as first secretary of the Communist Party of Cuba to Raul, who turned 75 in June and who has been taking on a more public profile in recent weeks. In power since the triumph of the Cuban revolution on Jan. 1, 1959, Castro has been the world's longest-ruling head of government. Only Britain's Queen Elizabeth, crowned in 1952, has been head of state longer. The "maximum leader's" ironclad 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. Over nearly five decades, hundreds of thousands of Cubans have fled Castro's rule, many of them settling just across the Florida Straits in Miami. Castro rose to power after an armed revolution he led drove out then-President Fulgencio Batista. The United States was the first country to recognize Castro, but his radical economic reforms and rapid trials of Batista supporters quickly unsettled U.S. leaders. Washington eventually slapped a trade embargo on the island and severed diplomatic ties. Castro seized American property and businesses and turned to the Soviet Union for military and economic assistance. On April 16, 1961, Castro declared his revolution to be socialist. The following day, he humiliated the United States by capturing more than 1,100 exile soldiers in the Bay of Pigs invasion. The world neared nuclear conflict on Oct. 22, 1962, when President John F. Kennedy announced there were Soviet nuclear missiles in Cuba. After a tense week of diplomacy, Soviet leader Nikita Krushchev removed them. Meanwhile, Cuban revolutionaries opened 10,000 new schools, erased illiteracy, and built a universal health care system. Castro backed revolutionary movements in Latin America and Africa. But former liberties were whittled away as labor unions lost the right to strike, independent newspapers were shut down and religious institutions were harassed. When social pressures increased, Castro provided a safety valve. In 1980, people desperate to leave the island poured into foreign embassies and the Cuban leader let 125,000 countrymen flee to Florida by boat through Mariel port, west of Havana. When economic crisis sparked rioting in Havana in 1994, Castro opened Cuba's borders again, and an estimated 30,000 people took to the sea in rafts. With Cuba's economy in a tailspin after the loss of Soviet aid, Castro was forced to open up to foreign capitalists and allow limited private enterprise. But when the economy began recovering in the late 1990s, Castro reasserted control and stifled private business. Castro continually resisted U.S. demands for multiparty elections and an open economy despite American laws tightening the embargo in 1992 and 1996. He characterized a U.S. plan for American aid in a post-Castro era as a thinly disguised attempt at regime change and insisted his socialist system would survive long after his death. Fidel Castro Ruz was born in eastern Cuba, where his Spanish immigrant father ran a prosperous plantation. His official birthday is Aug. 13, 1926, although some say he was born a year later. Talk of Castro's mortality was long taboo on the island, but that ended June 23, 2001, when he fainted during a speech in the sun. Although Castro quickly returned to the stage, many Cubans understood for the first time that their leader would one day die. Castro shattered a kneecap and broke an arm when he fell after a speech on Oct. 20, 2004, but typically laughed off rumors about his health, most recently a 2005 report that he had Parkinson's disease. "They have tried to kill me off so many times," Castro said in a November 2005 speech about the Parkinson's report, adding he felt "better than ever." But the Cuban president also said he would not insist on remaining in power if he ever became too sick to lead: "I'll call the (Communist) Party and tell them I don't feel I'm in condition ... that please, someone take over the command."