Señor Castro, 78, tripped and fell as he returned to his seat after speaking for an hour during a graduation ceremony held at a mausoleum where the remains of the Argentine revolutionary Ernesto “Che” Guevara are buried.

He got up and was helped to a chair, surrounded by security guards who rushed to his aid. He apologised for “any suffering this may have caused”, perhaps recalling how when he last collapsed three years ago some in the audience burst into tears.

“I will do what is possible to recover as fast as possible, but as you can see I can still talk,” he said, sweating profusely in his heavy olive-green uniform. “Even if they put me in a cast, I can continue in my work.”

The incident gave rise to speculation about the health of Señor Castro, who came to power in a revolution in 1959 and has remained in office despite US attempts to bring down his Communist regime.

A medical examination yesterday confirmed that the ageing leader had suffered a broken left knee and a hairline fracture in his upper right arm, according to an official notice read on state television.

In the note, Señor Castro expressed his thanks “for the innumerable expressions of concern and solidarity from the people”.

It added: “His general health is good, and he is in excellent spirits. He asked for calm, considering that very soon he will be back in place.”

A photographer at the scene said that Señor Castro tripped on a concrete step after he walked down the stairs from the stage, then fell to the ground on his side, first hitting a knee and hip and then an elbow.

After encouraging those at the event not to let his misfortune interrupt their televised musical programme, Señor Castro was seen leaving Santa Clara, about a three-hour drive east of Havana, in his black Mercedes Benz sedan.

Although the fall does not appear to have been caused by ill-health, Señor Castro has in recent years begun to show signs of the wear and tear of 45 years at the helm of a country in a perpetual battle for political and economic survival.

His health is especially closely watched in Miami, where a million CubanAmerican exiles await his demise as the only hope of political reform on the island.

A large man, he is slower on his feet these days, and his speech has slurred noticeably. He occasionally loses his flow in speeches in momentary lapses of thought, something that would have been unthinkable in his earlier years.

Even so, he maintains a busy schedule that frequently includes late-night meetings with advisers and visitors. Debate over Cuba’s political succession, once virtually taboo, has in recent years become a more open subject, with Señor Castro publicly referring to his own mortality. Even so, the idea of his death still seems remote to many.

One popular joke on the island depicts Señor Castro receiving a birthday present from the Cuban Communist Party of a live dinosaur egg, which he promptly declines when a scientist explains that the creature has a life expectancy of 200 years. “It would make me so sad when it passes away,” Señor Castro remarks.

While several potential successors wait in line, the Cuban Constitution provides for Señor Castro’s younger brother, Raúl, head of the Armed Forces and the first Vice-President, to assume the presidency.

He is a man with far less charisma, no oratory and largely untested political skills. Some have questioned how long he might be able to hold the party — and the country — together.

THE STRANGE LIFE OF A REVOLUTIONARY

• In 1940 Castro wrote to President Roosevelt requesting a $10 bill. The White House sent back a reply but without the $10
• In 1944 he won the award for Cuba’s best all-round school athlete. Not long after, he was scouted by the Pittsburgh Pirates baseball team but chose politics instead
• His inaugural speech on coming to power lasted seven hours
• Operation Mongoose employed 400 CIA agents to try to assassinate him. One plan involved sneaking a powder into his shoes so that he would lose his famous beard
• Another plot hoped to exploit his love of scuba diving by contaminating a diving suit and presenting it as a gift
• In 1985 he temporarily gave up cigars for a national health campaign
• His favourite drink is aged rum

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