By Stephen Gibbs
BBC News, Havana
Mr Castro spoke to the crowd for a little under an hour
More than a million people have gathered in the Cuban capital to attend a government-organised rally marking International Labour Day.
In a speech President Fidel Castro condemned the US administration, accusing it of harbouring terrorists.
Thousands of Cubans poured into Havana's vast Revolution Square, wearing regulation red T-shirts, and waving small Cuban flags.
They were joined by hundreds of visiting socialist delegations.
The theme of the day was terrorism against Cuba, and specifically the case of one man - Luis Posada Carriles.
Attack on US
The Cuban-born militant and would-be assassin of Fidel Castro has been linked to a series of violent attacks on Cuban interests.
He is currently understood to be seeking asylum in the US.
Describing Posada Carriles as a monster, President Castro said that his case exposed the hypocrisy of the US war on terror.
He bitterly attacked the Bush administration, labelling it as both stupid and genocidal.
By contrast, the 78-year-old said that in Cuba things were going well and that the Cuban revolution was stronger than ever.
Last month, Cuba, buoyed by recent trade deals with both Venezuela and China, doubled the state minimum wage to around $10 a month.
Fidel Castro has delivered no less than 11 major speeches in the last three weeks, this one was relatively briefly by his standards - he was at the podium for just over an hour.