Fidel Castro, who has defied the United States for nearly half a century while wielding absolute power over this island 90 miles south of Florida, remained out of sight Tuesday after undergoing intestinal surgery and temporarily turning over power to his brother Raul.

The surprise announcement that Castro had been operated on to repair a “sharp intestinal crisis with sustained bleeding” stunned Cubans on the island and in exile, and marked the first time that Castro, two weeks away from 80th birthday, had relinquished power in 47 years of rule.

People went about their business as normal on the streets of Havana early Tuesday, standing in line for buses to school and work, and jogging along the city’s famous Malecon seawall.

Some government work centers called workers to participate in outdoor political gatherings later Tuesday to express their support for Fidel Castro.

The news came Monday night in a statement read on state television by his secretary, Carlos Valenciaga. The message said Castro’s condition was apparently due to stress from a heavy work schedule during recent trips to Argentina and eastern Cuba. He did not appear on the broadcast.

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

Raul Castro, the 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 Castro become permanently incapacitated.

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.”

On Monday, before Castro’s illness was announced, President Bush was in Miami and spoke of the island’s future.

“If Fidel Castro were to move on because of natural causes, we’ve got a plan in place to help the people of Cuba understand there’s a better way than the system in which they’ve been living under,” he told WAQI-AM Radio Mambi, a Spanish-language radio station. “No one knows when Fidel Castro will move on. In my judgment, that’s the work of the Almighty.”

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

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, Hanauer said. The condition involves weakened spots in the intestinal lining that form pouches, which can become inflamed and provoke bleeding.

Fidel Castro 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.”

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.

She added that opposition members worried they could be targeted for repression during a government change, especially if authorities fear civil unrest.

Venezuelan President Hugo Chavez, Castro’s strongest international ally, expressed distress during a visit to Vietnam. He said he called the Cuban leader’s office after hearing the news.

“We wish President Fidel Castro will recover rapidly. Viva Fidel Castro!”

Chinese President Hu Jintao also sent a message of good wishes to Castro, the official Xinhua News Agency said.

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.

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.

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.”