Accompaning the photos in the Juventud Rebelde newspaper was a statement attributed to the Cuban president that said: "I feel very happy. For all those who care about my health, I promise to fight for it."
Although Castro was pictured in apparently good health, the road to recovery is not yet over.
The statement said: "To say that the objective stability has considerably improved is not making up a lie.
"To affirm that the recovery period will take a short time and that there is no risk would be absolutely incorrect."
However, the leader urged his public to be optimistic and be ready to face any adverse news.
"To the people of Cuba, infinite gratitude for your loving support. The country is marching on and will continue marching on perfectly well," the statement said.
Castro's brother, Raul, also made his first public appearance on Sunday since becoming acting president.
Raul Castro met Hugo Chavez, the Venezuelan president, with a military salute and a hug at Havana airport.
It was the Cuban defence minister's first appearance since his brother temporarily ceded power to him on July 31 due to surgery to stop intestinal bleeding.
Chavez, a top ally, flew to Havana to mark the elder Castro's birthday.
Details of Castro's health are treated as a state secret but on Saturday the communist party's daily newspaper, Granma, reported that he was "up and about".
"Someone who visited the commandante ... said he witnessed how the head of the revolution, after receiving a little physical therapy, walked in the room and later, sitting in a chair, engaged in an animated conversation," Granma reported.
Raul Castro became acting
president on July 31
No one knows if he will make an appearance or a speech on his birthday on Sunday but Cubans are being asked to carry out voluntary work in tribute to the president.
Official media called for "a huge productive day this Sunday as a dignified present to Fidel and the motherland".
Cubans are also being urged to donate blood and clean up their neighbourhoods.
Dozens of musicians are performing on Saturday night on the "Anti-Imperialist Stage" opposite the US diplomatic mission on Havana's Malecon seafront boulevard. They plan to sing "Happy Birthday" to Castro.
Hugo Chavez (L) said he will visit
Castro on his 80th birthday
Chavez said he would give
Castro a dagger and a cup carried by 19th century revolutionary hero Simon Bolivar as gifts.
"Since I already drank coffee from the cup, tomorrow I will take it to Fidel Castro as a present. Fidel, now you know, there will be no surprise tomorrow, this is your present," Chavez said.
Chavez also promised that he would arrive in Cuba with a "good cake" for Castro.
Several days of parties, concerts and conferences were also planned but they have been postponed until December 2, the 50th anniversary of Castro's landing in eastern Cuba to launch the guerrilla war that later brought him to power.