Friends and allies of Fidel Castro, the founding leader of communist Cuba and its former president, are celebrating his 90th birthday.

The revolutionary leader looked frail as he attended the celebrations on Saturday in the capital Havana with his brother, Raul Castro, and the Venezuelan President Nicolas Maduro.

"I want to express my deepest gratitude for the shows of respect, greetings and praise that I've received in recent days, which give me strength to reciprocate with ideas that I will send to party militants and relevant organisations," Castro wrote in a letter about his birthday.

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

The country has a month of festivities planned to honour the former president, including photography exhibitions and documentaries - as well as a Guinness World Record breaking 90-metre cigar to represent each year of his life.

Fidel Castro, centre, sits next to Cuba's President Raul Castro and Venezuelan President Nicolas Maduro during a cultural gala to celebrate his 90th birthday [Reuters]

Source: Al Jazeera and agencies