Bolivia President Evo Morales praised Castro as an "example of international solidarity," in a message to the father of Cuba's communist revolution, according to state media.
Another congratulatory note came from Nicaraguan president Daniel Ortega, who described him as "a relentless human guide."
"More than a political leader, he incarnates a historic moment where there was a choice to save humanity or not," Brazilian painter Helder Becerra was quoted as saying in the weekly Trabajadores.
Becerra has mounted an art show in Havana paying homage to Castro on his birthday.
Fidel Castro. Photo: AFP
Cuba's official newspapers also published congratulatory messages from Cubans and extracts from books extolling the merits of the man who led Cuba for a half century.
And across the island, groups of youths prepared tributes and concerts, particularly in Castro's hometown of Biran, in south-east Cuba about 650 kilometers from Havana.
A traditional birthday cake was crafted at the Palace of the Pioneers in Havana -- where Castro has often celebrated past birthday with his children.
But Castro was unlikely to taste the confection, a treat he has foregone since the serious health crisis in 2006 that led him to give up power.
During his decades in power, Castro fiercely guarded his privacy, to the point that many Cubans do not even know the name of his wife, Daliea Soto del Valle, and of the five children they had together.
Since turning over power to his brother in 2006, "el Comandante," as the Cubans call him, began celebrating his birthdays with family.
The aging leader has retired even further from his public persona this year -- he has not appeared in public since April and he has not published his "reflections" on international issues since mid-June.
Born August 13, 1926 to a prosperous Spanish immigrant landowner and a Cuban mother who had been his housekeeper, Castro was the hero of the island's revolution that swept Fulgencio Batista from power in 1959.
A communist icon at the height of the Cold War, the unrepentant anti-American and anti-imperialist leader is seen by supporters as a great revolutionary and by critics as a merciless dictator.