An estimated one million people have taken part in a march through Havana led by veteran President Fidel Castro.
The huge march passed within earshot of US diplomats
Waving tiny national flags they passed along the Cuban capital's waterfront to protest against new US sanctions.
Mr Castro set the tone with a speech which accused Washington of pursuing "world tyranny" and vowed to prevent Cuba becoming a US "neo-colony".
President George W Bush tightened the US embargo on Cuba last week in an effort to weaken Mr Castro's power.
The BBC's Stephen Gibb in Havana said the march took seven hours to pass the building where US diplomats are based in the city.
They shouted "Bush, you are a fascist!" and "down with imperialism!", our correspondent said.
Some had posters showing the US president in full Nazi uniform, complete with Adolf Hitler moustache, while others handed out images of abused Iraqi prisoners with the inscription "never in Cuba".
In an address to the demonstrators, Mr Castro said the US president had no right to lecture anyone on such things as democracy or human rights.
He likened himself to a Roman gladiator prepared to fight to the death with honour.
Our correspondent says that having shut many of its hard currency stores in the last few days in an apparent emergency bid to conserve resources, Cuba is giving the impression that it is getting on a war footing.
US officials say military action is out of the question and accuse Mr Castro of deliberately misleading the Cuban people to cover up his own failures.
Since the dollar was legalised in the early 1990s, dollar stores have become the only places for Cubans to buy not just luxuries but some essentials too, particularly clothes.
Cuban dissidents have warned that Mr Bush's declared aim to speed up political change in Cuba has given Mr Castro the perfect opportunity to create a sense of crisis in the country.
Amongst the new US measures, which add to sanctions imposed since the Cuban Revolution of 1959, are:
- the limitation of family visits between the US and Cuba to one in every three years
the restriction of cash remittances from expatriates to their relatives in Cuba - a key source of Cuban revenue
worth about $1.2bn annually
the use of US military aircraft to broadcast pro-democracy radio and TV television programmes into Cuba, circumventing Havana's jamming of signals.