Castro with Hassan Khomeini

Dressed in his trademark olive green uniform and a less typical pair of white sneakers, the Cuban president vigorously waved a small Cuban flag as he marched past the only US diplomatic mission in the country on the sea-front Malecon boulevard.

The Cuban authorities began bussing people into the centre of Havana from all parts of the city and the surrounding province in the early hours of the morning.

BBC correspondent Daniel Schweimler says the streets were a sea of people for as far as the eye could see, walking slowly, waving flags, singing and chanting.

"Down with the genocidal blockade!" the marchers chanted, referring to the four-decade US trade embargo against the communist island.

'Patriotic heroes'

"Down with the murderous law!" they shouted in protest at US immigration policies that Havana says encourage Cubans to risk their lives on dangerous sea journeys in the hope of gaining American citizenship.

Socialist Cuba says the slogan, to the chagrin of the USA

"Free the patriotic heroes!" yelled the marchers, many of whom wore white T-shirts emblazoned with pictures of five Cuban agents convicted in Miami and awaiting sentencing on espionage charges.

Havana has demanded their release, saying they were merely gathering information about anti-Castro groups to prevent violent attacks.

Marching to Castro's left was 29-year Hassan Khomeini, grandson of the leader of Iran's 1979 Islamic revolution, the late Ayatollah Ruhollah Khomeini.

Castro challenge

National Rebellion Day, as 26 July is known in Cuba, was the day in 1953 that a young Fidel Castro, his brother Raul and 150 followers attacked the Moncada military barracks in the east of the country.

The attack was a failure, with half the combatants being killed, and Fidel and Raul captured.

During his trial, Fidel Castro gave his famous "History will absolve me" speech which laid the foundations for his revolution.

Six years later, the US-backed President Fulgencio Batista fled, and a triumphant Fidel Castro marched into Havana alongside his bearded revolutionaries.

But after 42 years in power, Castro is constantly faced with the challenge of keeping his revolution alive and relevant to the Cuban people.

This stage-managed march was the latest attempt to show both Washington and Cuba that he is still very much in control.

Trade embargo

US President George W Bush's public pledges to get tough on Castro by tightening the 40-year-old trade embargo have only fuelled Castro's militancy, and this was perhaps an underlying reason for calling for the largest march in Cuba's history to mark National Rebellion Day.

The Republican-led House of Representatives voted on Wednesday to lift restrictions on Americans travelling to Cuba.

But Mr Bush wants to keep the restrictions at their current level until the communist authorities in Havana release many of their political prisoners.

A similar measure to lift restrictions was overturned by the Senate last year.