The Web site of the state-run newspaper, "Granma," posted Castro's resignation letter on Monday night.
In it, he wrote, "To my dearest compatriots, who have recently honored me so much by electing me a member of the Parliament where so many agreements should be adopted of utmost importance to the destiny of our Revolution, I am saying that I will neither aspire to nor accept, I repeat, I will neither aspire to nor accept the positions of President of the State Council and Commander in Chief."
He said the responsibility would require more mobility and dedication than he was physically able to offer, though he will continue to offer ideas and writings.
"This is not my farewell to you," he continued. "My only wish is to fight as a soldier in the battle of ideas. I shall continue to write under the heading of 'Reflections by comrade Fidel.' It will be just another weapon you can count on. Perhaps my voice will be heard. I shall be careful."
Castro's brother Raul, 76, has been serving as president since Fidel underwent intestinal surgery in the summer of 2006. Raul is expected to officially take the leadership post over the weekend.
While Raul Castro has made few changes to Cuba's economic policies so far, his leadership has raised expectations that Cubans will soon be allowed to freely buy and sell their homes, travel abroad and stay at hotels and beaches where only foreigners can go, according to Reuters.
"The challenges we have ahead are enormous, but may no one doubt our people's firm conviction that only through socialism can we overcome the difficulties and preserve the social gains of half a century of revolution," he said late last year.
The younger Castro also has reached out to Cuba's long-time enemy the United States by saying in July that Havana was open to talks to end more than four decades of hostility, but only when President Bush has left the White House, Reuters reported.
While traveling in Rwanda, President Bush said he hoped the end of Castro's presidency would usher in democracy for the Cuban people.
"They're the ones who suffered under Fidel Castro," he told a news conference, according to the Associated Press. "They're the ones who were put in prison because of their beliefs. They're the ones who have been denied their right to live in a free society. So I view this as a period of transition and it should be the beginning of the democratic transition in Cuba."
U.S. State Department officials offered similar sentiments, but they emphasized that such changes would not come quickly under Raul Castro, who is considered a continuation of the Castro regime.
Cuban exiles living in Miami also welcomed the announcement of Castro's resignation, but expressed little optimism that the transition would bring major changes or democracy to the communist nation.
"I hope this is the beginning of the end of the system, but we have to wait," said chemist Omar Fernandez, who left Cuba for the United States six years ago, quoted the AP.
"Even though this is great news for Cubans and for me personally, but I don't think anything is going to change," said Jose Miranda, 46, according to the AP.