HAVANA — An ailing Fidel Castro resigned as Cuba’s president today after nearly a half-century in power, saying he was retiring and will not accept a new term when the new parliament meets Sunday.

“I will not aspire to nor accept — I repeat, I will not aspire to nor accept — the post of President of the Council of State and Commander in Chief,” read a letter signed by Mr. Castro published early today in the online edition of the Communist Party daily Granma.

The announcement effectively ends the rule of Mr. Castro, 81, after almost 50 years, positioning his 76-year-old brother Raul Castro for permanent succession to the presidency. Fidel 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 Mr. 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.

A new National Assembly was elected in January, and will meet for the first time Sunday to pick the governing Council of State, including the presidency that Fidel Castro has held since the assembly’s 1976 creation. Before 1976, Mr. Castro was president under a different government structure, and previously served as prime minister.

There had been wide speculation about whether Mr. Castro, Cuba’s unchallenged leader since 1959, would continue as president.

“My wishes have always been to discharge my duties to my last breath. That’s what I can offer,” Mr. Castro wrote. But, he continued, “it would be a betrayal to my conscience to accept a responsibility requiring more mobility and dedication than I am physically able to offer. This I say devoid of all drama.”

Mr. 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 America.

The resignation opens the path for Raul Castro’s succession to the presidency, and the full autonomy he has lacked in leading a caretaker government. The younger Mr. Castro has raised expectations among Cubans for modest economic and other reforms, stating last year that the country requires unspecified “structural changes” and acknowledging that government wages that average about $19 a month do not satisfy basic needs.

As first vice president of Cuba’s Council of State, Raul Castro was his brother’s constitutionally designated successor and appears to be a shoo-in for the presidential post when the council meets Sunday. More uncertain is who will be chosen as Raul Castro’s new successor, although the 56-year-old council vice president, Carlos Lage, who is Cuba’s de facto prime minister, is a strong possibility.

In the pre-dawn hours, most Cubans were unaware of Castro’s message. Havana’s streets were quiet, and there was not even any movement at several party-run neighborhood watch groups known as Revolutionary Defense Committees in Old Havana.

It wasn’t until 5 a.m., several hours after Mr. Castro’s message was posted on the Internet, that official radio began reading the missive to early risers across the island.

President Bush was notified of Mr. Castro’s resignation by his national security adviser while traveling in Africa, a spokesman, Gordon Johndroe, said.

Mr. Castro rose to power on New Year’s Day 1959 and reshaped Cuba into a communist state 90 miles from American shores. The fiery guerrilla leader survived assassination attempts, a CIA-backed invasion and a missile crisis that brought the world to the brink of nuclear war. Ten American administrations tried to topple him, most famously in the disastrous Bay of Pigs invasion of 1961.

His ironclad rule ensured Cuba remained communist long after the breakup of the Soviet Union and the collapse of communism across Eastern Europe.

Monarchs excepted, Mr. Castro was the world’s longest ruling head of state.

“The adversary to be defeated is extremely strong,” Mr. Castro wrote today, referring to America. “However, we have been able to keep it at bay for half a century.”

Raul Castro had long been his brother’s designated successor. The longtime defense minister had been in his brother’s rebel movements since 1953 and spent decades as no. 2 in Cuba’s power structure.

America, bent on ensuring neither brother is in power, built a detailed plan in 2005 for American assistance to ensure a democratic transition on the island of 11.2 million people after Fidel Castro’s death. But Cuban officials insisted there would be no transition, saying the island’s socialist political and economic systems would outlive Mr. Castro.

Mr. Castro’s supporters admired his ability to provide a high level of health care and education for citizens while remaining fully independent of America. His detractors called him a dictator whose totalitarian government systematically denied individual freedoms and civil liberties such as speech, movement, and assembly.

America was the first country to recognize Mr. Castro after his guerrilla movement drove out the president at the time, Fulgencio Batista, in 1959. But the two countries soon clashed over Mr. Castro’s increasingly radical path. Mr. Castro seized American property and businesses and invited Soviet aid.

On April 16, 1961, Mr. Castro declared his revolution to be socialist. A day later, he defeated the CIA-backed Bay of Pigs invasion.

America squeezed Cuba’s economy and the CIA plotted to kill Mr. Castro. Undaunted, the Cuban president supplied troops and support to revolutionaries in Africa and Latin America.

Hostility over Cuba reached its peak on October 22, 1962, when President Kennedy announced there were Soviet nuclear missiles in Cuba. After a tense week of diplomacy, the Soviet premier, Nikita Krushchev, pulled out the weapons.

With the end of the Cold War in the early 1990s, Mr. Castro eventually made peace with many governments that once shunned him. Pope John Paul II visited the island in January 1998.

The loss of Soviet aid plunged Cuba into financial crisis, but the economy slowly recovered in the late 1990s with a tourism boom.

Mr. Castro later reasserted control over the economy, stifling the limited free enterprise tolerated during more difficult times.

Fidel Castro Ruz was born in eastern Cuba, where his Spanish immigrant father ran a prosperous plantation. His official birthday is August 13, 1926, although some say he was born a year later.

He attended Roman Catholic schools and the University of Havana, where he received law and social science degrees.

Mr. Castro launched his revolutionary battle as a young man, organizing an unsuccessful July 26, 1953 attack on a military barracks in the eastern city of Santiago.

Later freed under a pardon, Mr. Castro went to Mexico and organized a rebel army that returned to Cuba and rallied support in the Sierra Maestra mountains. His rebels took power when Batista was forced to flee.

Entering Havana triumphantly, Mr. Castro declared: “Power does not interest me, and I will not take it.”