Fidel Castro said in a statement read on state television Tuesday that his health is stable and he is in good spirits after undergoing surgery to stop intestinal bleeding.
The Venezuelan government said in a statement that Fidel's recovery was "advancing positively," citing information from Cuba's government without providing more details. A leftist Argentine lawmaker said aides told him Castro was resting peacefully after successful surgery.
Cuban state television announced Monday night that Castro had been operated on to repair a "sharp intestinal crisis with sustained bleeding" and had temporarily turned over presidential powers his brother Raul, shocking Cubans on the island and in exile. It was the first time that Castro, two weeks away from 80th birthday, had relinquished power in 47 years of absolute rule.
Castro said in a letter read live by his secretary, Carlos Valenciaga, that extreme stress "had provoked in me a sharp intestinal crisis with sustained bleeding that obligated me to undergo a complicated surgical procedure." Castro did not appear on the broadcast.
Defense Minister Raul Castro, who turned 75 in June, has for decades been the constitutional successor. He did not appear on television and made no statement of his own.
Cubans went about business as usual in Havana Tuesday, standing in line for buses to school and work, and jogging along the city's famous Malecon seawall.
Some government work centers summoned employees to participate in outdoor political gatherings to express support for Fidel. Dozens of workers at one gathering waved small Cuban flags and shouted: "Long live Fidel!"
"There is no one else like him," said Osmar Fernandez, 27, drinking rum at a cafe. "I want Fidel to live for 80 more years."
Government opponents said the move gave them hope for eventual openings in Cuba's political and economic systems.
"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 ... the top leaders will be obligated to consult each other [rather than be ruled by one man]."
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.
Looking thin and somewhat weary, Fidel Castro last appeared in public Wednesday in eastern Cuba as he marked the 53rd anniversary of his July 26 barracks assault that launched the revolution.
"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."
It is official U.S. policy to "undermine" Cuba's planned succession to Raul Castro.
On Monday, before Castro's illness was announced, U.S. President George W. 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.
The Venezuelan government said in a statement that it had "received with satisfaction the information from Cuban authorities according to which the recovery process of President Fidel Castro is advancing positively." The statement did not give specifics about Castro's condition.
A leftist Argentine lawmaker, Miguel Bonasso, said he called Castro aides Monday night and was told the surgery "was successful" and the leader was resting.
Ongoing intestinal bleeding can be serious and potentially life-threatening, said Dr. Stephen Hanauer, gastroenterology 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, the noontime news on Cuban state television led with Castro's letter, followed by interviews with ordinary Cubans expressing their confidence in his recovery. One item showed Bolivians rallying for Castro at the Cuban Embassy in La Paz.
The view from the United States in Cuba was limited to the electronic news ticker at the U.S. diplomatic mission, which read: "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.
She added that opposition members worried they could be targeted for repression during a government change — especially if authorities fear civil unrest.
"For Fidel Castro to delegate functions of state, it must be something extremely serious," Chavez said, according to a government statement.
Yet he also said he was optimistic that within a few weeks the Cuban leader "will be once again as he has been and will always be: with us," declaring "Viva Fidel Castro!"
Chinese President Hu Jintao also sent a message of good wishes to Castro, the official Xinhua News Agency said.
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.
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."
The Associated Press contributed to this report.