HAVANA — An ailing, 81-year-old Fidel Castro resigned as Cuba's president Tuesday after nearly a half-century in power, saying he will not accept a new term when parliament meets Sunday.

The end of Castro's rule — the longest in the world for a head of government — frees his 76-year-old brother Raul to implement reforms he has hinted at since taking over as acting president when Fidel Castro fell ill in July 2006. President Bush said he hopes the resignation signals the beginning of a democratic transition.

"My wishes have always been to discharge my duties to my last breath," Castro wrote in a letter published Tuesday in the online edition of the Communist Party daily Granma. But, he wrote, "it would be a betrayal to my conscience to accept a responsibility requiring more mobility and dedication than I am physically able to offer."

In the pre-dawn hours, most Cubans were unaware of Castro's message, and Havana's streets were quiet. It wasn't until 5 a.m., several hours after Castro's message was posted on the internet, that official radio began reading the missive to early risers.

By sunrise, most people headed to work in Havana seemed to have heard the news, which they appeared to accept without obvious signs of emotion. There were no tears or smiles as Cubans went about their usual business.

"He will continue to be my commander in chief, he will continue to be my president," said Miriam, a 50-year-old boat worker waiting for the bus to Havana port. "But I'm not sad because he isn't leaving, and after 49 years he is finally resting a bit."

Castro temporarily ceded his powers to his brother on July 31, 2006, when he announced that he had undergone intestinal surgery. Since then, the elder Castro has not been seen in public, appearing only sporadically in official photographs and videotapes and publishing dense essays about mostly international themes as his younger brother has consolidated his rule.

There had been widespread speculation about whether Castro would continue as president when the new National Assembly meets Sunday to pick the country's top leadership. Castro has been Cuba's unchallenged leader since 1959 — monarchs excepted, he was the world's longest ruling head of state.

Castro said Cuban officials had wanted him to remain in power after his surgery.

"It was an uncomfortable situation for me vis-a-vis an adversary that had done everything possible to get rid of me, and I felt reluctant to comply," he said in a reference to the United States.