Despite official denials, speculation is still rife that Castro is dying, or too sick to return to the presidency after undergoing emergency surgery in late July.
The illness forced Castro to cede power temporarily to his younger brother, the defence minister Raul, days before his 80th birthday.
Carlos Lage, Cuba's vice president, said on Friday that Castro was getting better and would continue leading the communist island.
"Fidel is recovering, we will have him among us, he will continue leading," Lage told 5,000 people at Havana's Karl Marx Theatre.
"We will ask him to do it for several years."
Lage also defended Cuba's one-party communist system, saying it would outlast Castro.
"I feel that socialism in Cuba is irreversible, because with our efforts today and tomorrow, we are going to make it irreversible."
Castro has attended none of his birthday festivities, saying in a statement read at an opening gala on Tuesday that his doctors told him he was not yet ready for "such a challenging engagement".
It was left to Raul to welcome international guests, including the presidents of Bolivia and Nicaragua, Evo Morales and Daniel Ortega.
|Cuba's vice president says the country's |
coummunist system will outlive Castro
The tributes to Castro also became a celebration of the resurgence of the left in Latin America.
"We have Venezuela, Cuba and soon Ecuador and Nicaragua," Morales told thousands of guests at the convention center.
"We want to make alliances with countries in the Middle East to bring an end to the United States empire!"
Minutes before Morales took the podium, Ortega announced that he had something special for Castro.
"I give you as a birthday gift the victory of the Sandinista Front in Nicaragua," he said.
'Fountain of ideas'
Venezuela's foreign minister, Nicolas Maduro - appearing in place of Castro's main ally, Hugo Chavez, the Venezuelan president - said the Cuban leader "has in all of these battles been a fountain of ideas, of life".
Chavez, who couldn't attend the event in Havana because he is running for re-election on Sunday, has said if he wins, he would dedicate his victory to Castro.
Colombian Nobel laureate Gabriel Garcia Marquez expressed optimism about Castro's health upon his arrival in Havana, the Communist Party newspaper Granma reported.
"What makes me most happy about being able to come now for the 80th birthday of Fidel is that I will be back for his 100th."