By ANTHONY DePALMA

HAVANA — Fidel Castro, bedridden for 19 months, on Tuesday gave up the almost unlimited power he has wielded in Cuba for nearly 50 years, but whether the surprise announcement represented a historic change or a symbolic political maneuver remained unclear.

It is expected that his brother Raúl, 76, will be officially named president, and some experts consider him more pragmatic.

Raúl Castro has talked about bringing more accountability to government and possibly working to improve relations with the United States. But since taking over temporarily in the summer of 2006, he has largely operated in his brother’s shadow, and, except for facilitating huge investments by Canadian and European resort developers here, he has brought about little change.

Under Cuba’s Constitution, a newly chosen legislative body, the National Assembly, is scheduled to select a 31-member Council of State on Sunday. In turn, the new council will pick the next president. Fidel Castro said he would not accept the position even if it were offered to him.

In a letter of resignation read over early morning radio and television programs across the country, the 81-year-old Mr. Castro — who has appeared frail in the few videos released by the Cuban government — was said to be too ill to continue as head of state and would not stand in the way of others who were ready to take over, a sentiment he first expressed last December.

Experts on Cuban politics say the decision on a successor remains in the hands of the Castro brothers and their inner circle, many of whom hold cabinet positions. Others said that a younger president could be brought in or that the posts of prime minister and president could be divided between Raúl Castro and one of the ministers.

It was not clear what role, if any, Fidel Castro would play in a new government, or whether he would retain other powerful positions, including head of the Communist Party. But he signaled that he was not yet ready to completely exit the stage.

It is not even certain that Mr. Castro was well enough to actually write the letter of resignation. Doubts have arisen over his health and whether he could have written a string of essays that have been published over the last year and a half in Granma, the Communist Party organ.

“I am not saying goodbye to you,” said Mr. Castro in the letter written under his name and addressed to the Cuban people. “I only wish to fight as a soldier of ideas.”

The confusion of analysts in both Cuba and the United States about the extent to which Mr. Castro would withdraw from day-to-day government operations or continue to wield power from behind the scenes was reflected in the mix of opinions of people from the luxury beaches at the seaside resort of Varadero to the central park of Old Havana.

There was little evidence in the streets of the capital and in other cities to suggest that a monumental change was taking place in the Cuban hierarchy. But that could be because the accrued experience of state security officials made open demonstrations unlikely and assured that most reactions to the news would remain as covert as a high-five between passing friends on a crowded street, an act viewed in Havana.

Cuba’s leading dissident tried to dampen expectations.

“This isn’t news,” said the dissident, Elizardo Sánchez, after learning from friends that Mr. Castro was ceding power. “It was expected and it does nothing to change the human rights situation, which continues to be unfavorable, or to end the one-party state. There’s no reason to celebrate.”

The pace of ordinary Cuban life continued.

In Varadero, workers collected garbage and cleaned pools as they normally would. On the highway, workers whitewashed barriers. In the seaside city of Matanzas, Eliana Lopez, a 55-year-old transportation inspector who had heard the news on her way to work, said she expected the revolution to continue, with change coming slowly but surely.

“There is no surprise,” Ms. Lopez said. She added, “This is the correct decision,” referring to Mr. Castro’s declaration.

In Havana, an older generation of Cubans who maintained their admiration for Mr. Castro and his revolution, despite the crumbling condition of the city, were disappointed. Alba, 67, a retired nurse who, like many Cubans, was afraid to give her full name, told Agence France-Presse that she had expected Mr. Castro to be president for life and to “die with his boots on.”

However, members of a younger generation who had grown tired of what they saw as promises for a better life that never materialized, said they hoped there might be significant change, although their hopes were based more on wishful thinking than on a realistic view of the political situation.

“It took a long time, but he finally said it and that’s fantastic news,” said Armando, a 28-year-old student who said it was about time Mr. Castro let go of power.

And there were many Cubans like Evelyn, 45, who neither cheered nor cried at the news that Mr. Castro would step aside.

“I am not surprised and I don’t feel sad either,” Evelyn said. “Everyone knew he was going to resign. Fidel is an athlete of politics. He knows what he does.”

In trying to assess the future, political analysts pointed to signs that Raúl Castro had a pragmatic streak. As acting president, he has encouraged more debate about Cuba’s economic woes, sponsoring a series of town-hall-style meetings last fall to allow people to speak freely about their economic woes and the limits on their rights to travel.

He has also brought up issues his brother never addressed. He has lambasted farmers for being inefficient. He has criticized the high cost of milk. He has acknowledged that the salaries the Cuban government pays are woefully low and do not meet the minimum needs of a family. He has criticized corruption, even letting state-controlled newspapers publish investigative articles about the looting and bad management practices at many state-controlled companies.

The younger Mr. Castro also has a reputation as a manager who demands results from his cabinet members. Unlike Fidel, who liked to manage every detail of government himself, Raúl has delegated authority and held his cabinet ministers accountable.

After decades during which Mr. Castro’s grip seemed unbreakable, uncertainties arose in July 2006. Mr. Castro, suffering from an undisclosed abdominal ailment, had emergency surgery and temporarily handed over power to his brother.

In Tuesday’s letter, which was also published in Granma, Mr. Castro said he did not step down earlier to avoid dealing a blow to the Cuban government before the people were ready for a traumatic change “in the middle of the battle” with the United States over control of the Cuba’s future.

“To prepare the people for my absence, psychologically and politically, was my first obligation after so many years of struggle,” he said. More than 70 percent of Cubans were born after he seized power in 1959.

President Bush, the 10th president to cross swords with Mr. Castro, has tried to tighten the longstanding United States embargo and increase international pressure on Cuba during his time in office, restricting family visits to once every three years and putting a cap on the amount of remittances Cuban-Americans can send to relatives in Cuba.

Mr. Bush, traveling in Rwanda on a tour of Africa, reacted to Mr. Castro’s decision to give up power by saying that the resignation should be the beginning of a democratic transition in Cuba leading to free elections. “The United States will help the people of Cuba realize the blessings of liberty,” he said.

Mr. Bush called for Cuba to release political prisoners and to begin building “institutions necessary for democracy that eventually will lead to free and fair elections.”

Analysts in the United States say that even after Fidel Castro dies, Raúl Castro, as president, would still find himself under tremendous pressure to sustain his brother’s legacy, while at the same time working to break it down and provide a measure of economic and political freedom for the Cuban people.

If the National Assembly on Sunday unexpectedly selects someone other than Raúl Castro — the names of the Assembly’s president, Ricardo Alarcón, Vice President Carlos Lage and Foreign Secretary Felipe Peréz Roque are mentioned frequently — it could represent a sign that Cuba is more eager to accelerate change than is widely believed, including an improved relationship with Washington.

The United States has stated that it would not negotiate with any Cuban government headed by either Fidel or Raúl Castro.

James C. McKinley Jr. contributed reporting from Mexico City, and Marc Lacey from Cancún, Mexico.