Cubans were stunned when Castro's secretary read a letter on state television Monday night announcing their leader was temporarily turning over power to his younger brother, the island's defense minister and the president's designated successor.

In the letter, Castro, who turns 80 on Aug. 13, said doctors operated to repair a "sharp intestinal crisis with sustained bleeding." Neither Castro brother was shown.

Alarcon said Castro made a point of delegating all responsibilities when his doctors told him to rest - a decision he said was made by a man "who was completely conscious and able to adopt these resolutions."

Castro had been seen frequently in recent days, delivering speeches in eastern Cuba during a revolutionary holiday and making waves at a trade summit in Argentina. Those back-to-back trips and the resulting stress "ruined" his health, according to his letter.

"It's so surprising, because in Argentina he gave off such a strong political image and looked quite vital," said Rafael Marti, a businessman from Spain visiting Cuba with his wife. He said he didn't expect rapid change on the island 90 miles south of Florida.

Castro, who took control of Cuba in 1959, has resisted repeated U.S. attempts to oust him as well as demands for western-style elections and an open economy.

Doctors in the United States said Castro's condition could be life-threatening but since the details of his symptoms were not released it was hard to say what caused the bleeding: severe ulcers, a colon condition called diverticulosis or - an outside possibility - cancer.

Castro seemed optimistic of recovery, asking in his letter that celebrations scheduled for his 80th birthday be postponed until Dec. 2, the 50th anniversary of Cuba's Revolutionary Armed Forces.

The leaders of China, Venezuela, Bolivia and Mexico wished Castro well.

Castro has been in power since the Jan. 1, 1959, triumph of the armed revolution that drove out Fulgencio Batista. He has been the world's longest-ruling head of government.

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.