The young Castro led his ragged band of revolutionaries to victory on 1 January, 1959, when US-backed dictator Fulgencio Batista fled the island.

One of Castro's first speeches in 1959

He has gone on to become one of the world's longest surviving leaders, despite constant US hostility, an economic boycott and several attempts to unseat him.

Cubans - struggling under economic crisis - are preparing to celebrate the anniversary with a solemn ceremony in Santiago de Cuba.

Patriotic displays and speeches, rather than fireworks, will be the order of the day in the city - considered the cradle of the revolution.

But BBC correspondent Tom Gibb says enthusiasm for the anniversary is lukewarm.

Hunger and prostitution

The country is increasingly divided between loyal supporters of the revolution and those disillusioned with changes since the fall of Cuba's Soviet backers.

Cubans now dance in Central Park - site of a key revolutionary battle

This decade has seen real hunger on the island and the return of prostitution and street crime.

State-controlled media and the one-party system remain almost absolute.

The government blames a tightened US embargo for its economic failures - the last sugar harvest took production back to the levels of the First World War. But opponents blame the dozens of government restrictions on free enterprise.

In recent years President Castro has presented Cuba as a lone beacon to the rest of the world, predicting the demise of global capitalism.

The veteran revolutionary, the darling of world's radical left, still inspires respect for standing up to the US.

But our correspondent says inside Cuba many believe the ideals which inspired the revolution 40 years ago are now struggling to survive.

Fighting 'Yankee imperialists'

The ruling Communist Party, which President Castro heads, has been using this week's anniversary to urge Cubans to keep faith in their socialist system into the 21st century.

Communist Party official Esteban Lazo Hernandez said: "For the revolutionaries of the country ... to have lived these 40 years is a privilege that many in the world would like to have.

"We have faced a variety of difficulties, due to the iron blockade (trade embargo) imposed by Yankee imperialism, but we ... will resist with audacity, intelligence and courage under the sure leadership of Fidel."