Former Cuban leader Fidel Castro (C), Cuba's President Raul Castro (L) and Venezuela's President Nicolas Maduro attend a gala for Fidel Castro's 90th birthday at the Karl Marx theatre in Havana, Cuba, August 13, 2016. [Photo/Agencies]

Fidel Castro thanked Cubans for their best wishes on his 90th birthday and criticized US President Barack Obama in a lengthy letter published by state media. He appeared but did not speak at a gala in his honor broadcast on state television.

"I want to express my deepest gratitude for the shows of respect, greetings and praise that I've received, which give me strength to reciprocate with ideas that I will send to party militants and organizations," he wrote on Saturday.

"Modern medical techniques have allowed me to scrutinize the universe," wrote Castro, who stepped down as Cuba's president 10 years ago after suffering a severe gastrointestinal illness.

Just after 6 pm, he was seen in footage on state television slowly approaching his seat at Havana's Karl Marx theater, clad in a white Puma tracksuit top and green shirt. He sat in what appeared to be a specially equipped wheelchair and watched a musical tribute with highlights from his decades in power.