An operation forced Fidel Castro to temporarily cede power to his brother for the first time in 47 years, abruptly ushering in a period of uncertainty in the communist island as the severity of his illness remained unknown.

Cuba"s President Fidel Castro (R) stands with his brother Raul during an event in Havana in this June 28, 2001 file photo. Castro underwent intestinal surgery on July 31, 2006 and delegated government functions provisionally to his younger brother Raul, the government said in a televised statement signed by the Cuban leader. (Stringer/Files - CUBA/Reuters)

Castro, less than two weeks away from his 80th birthday, did not appear on the live television broadcast Monday night in which his secretary read a letter from the Cuban leader. Castro"s message said he underwent surgery for gastrointestinal bleeding, apparently due to stress from a heavy travel and work schedule related to recent trips to Argentina and eastern Cuba.

Castro, who took control of Cuba in 1959, resisted repeated U.S. attempts to oust him and survived communism"s demise elsewhere, said he was temporarily handing over the presidency and the leadership of Cuba"s Communist Party to his brother, Raul.

Raul Castro, Cuba"s defense minister who turned 75 in June, also did not appear on television and made no statement on his own. For decades the constitutional successor to his brother, Raul Castro has assumed a more public profile in recent weeks.

Fidel Castro last appeared in public Wednesday as he marked the 53rd anniversary of his July 26 barracks assault that launched the revolution. The Cuban leader seemed thinner than usual and somewhat weary during a pair of long speeches in eastern Cuba.

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

The calm delivery of the announcement appeared to signal that there would be an orderly succession should Fidel become permanently incapacitated.

Cuban exiles celebrated in the streets of Miami, but Havana"s streets were quiet overnight as Cubans awaited further word on Castro"s condition.

It was unknown when or where the surgery took place, or where Castro was recovering.

Ongoing intestinal bleeding can be serious and potentially life-threatening, said Dr. Stephen Hanauer, gastroenerology chief at the University of Chicago hospitals. He said it was difficult to deduce the cause of Castro"s bleeding without knowing what part of the digestive tract was affected.

Ulcers are a common cause of bleeding in the stomach or upper intestine, while a condition called diverticulosis also can provoke bleeding in the lower intestine, especially in people over age 60, said Hanauer. He said this condition involves weakened spots in the intestinal lining that form pouches, which can become inflamed and provoke bleeding.

Fidel seemed optimistic of recovery, asking that celebrations scheduled for his 80th birthday on Aug. 13 be postponed until Dec. 2, the 50th anniversary of Cuba"s Revolutionary Armed Forces.

With Havana"s streets calm, an electronic news ticker at the U.S. diplomatic mission provided the only clue that something dramatic had occurred inside Cuba"s government: "All Cubans, including those under the dictatorship, can count on our help and support. We respect the wishes of all Cubans."

Waiters at a popular cafe in Old Havana were momentarily stunned by the news, but quickly returned to work.

"He"ll get better, without a doubt," said Agustin Lopez, 40. "There are really good doctors here, and he"s extremely strong."

But Martha Beatriz Roque, a leading Cuban government opponent in Havana, said she believed Castro must be gravely ill to have stepped aside — even temporarily.

"No one knows if he"ll even be alive Dec. 2 when he"s supposed to celebrate his birthday," she said in a telephone interview. She said opposition members worried they could be targeted for repression during a government change — especially if authorities fear civil unrest.

White House spokesman Peter Watkins said U.S. authorities were monitoring the situation: "We can"t speculate on Castro"s health, but we continue to work for the day of Cuba"s freedom."

Venezuelan President Hugo Chavez, Castro"s strongest international ally, was the first foreign leader to react to the news, expressing his distress during a visit to Vietnam. He said he called the Cuban leader"s office after hearing the news.

"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."

Across the Florida straits in Miami, exiles waved Cuban flags on Little Havana"s Calle Ocho, shouting "Cuba! Cuba! Cuba!" as drivers honked their horns. Over nearly five decades, hundreds of thousands of Cubans have fled Castro"s rule, many of them settling in Miami.

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.

Castro resisted U.S. demands for multiparty elections and an open economy and insisted his socialist system would long outlive him.

The son of a prosperous plantation owner, Castro"s official birthday is Aug. 13, 1926, although some say he was born a year later.

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.

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."

Associated Press writers Vanessa Arrington in Havana, Laura Wides-Munoz in Miami, Tran Van Minh in Hanoi, Vietnam, and Medical Writer Lindsey Tanner in Chicago contributed to this report.