US-based Forbes magazine ranked the Cuban president as the seventh wealthiest ruler in the world in its annual tally of the "Fortunes of Kings, Queens and Dictators".
The magazine estimated Castro's personal wealth to be $900 million - nearly double that of the of Britain's Queen Elizabeth II (at $500 million) and just below Prince Albert II of Monaco (estimated at $1 billion).
Denouncing the claim, Castro appeared in a four-hour long broadcast on Cuban television brandishing a copy of the magazine.
The story, he said, was "repugnant slander" by a capitalist publication.
With Communist Cuba's Central Bank governor at his side, Castro challenged Forbes to prove the allegation.
"If they can prove that I have a bank account abroad, with $900 million, with $1 million, $500,000, $100,000 or $1 in it, I will resign," he said at the end of a four-hour broadcast.
"It is so ridiculous to say I have a fortune of $900 million, a fortune with no heirs. What would I need all that money for, if I will soon be 80 years old?
The article also referred to rumours of Castro having "large stashes in Swiss bank accounts."
"I've been listening to this wickedness for nearly half a century - I don't pay much attention," said Castro, dressed in his trademark olive green military uniform.
Forbes magazine says Castro is
richer than Queen Elizabeth II
"Neither lies nor slander are worth anything."
Castro, in power since a leftist revolution in 1959, says his net worth is nil and that he earns only 900 Cuban pesos ($40) a month.
Kings and sheiks of the oil-rich Gulf Arab states still top the Forbes list, published in its May 22 edition.
Castro last year threatened to sue Forbes after the magazine included him on its 2005 list with an estimated fortune of $550 million.
The Forbes estimate includes state enterprises that the magazine assumes he controls in Cuba, among them the Havana Convention Centre, the Cimex retail conglomerate and a pharmaceutical company that exports vaccines.
Forbes said it assumed a portion of the profits of state companies goes to Castro and cited rumours of money stashed in Swiss bank accounts, but gave no details, saying former Cuban officials insist Castro has skimmed profits for years.
"What would I need all that money for, if I will soon be 80 years old?"
Several top Cuban officials also rejected the article during the broadcast.
Francisco Soberon, president of the Central Bank, said: "It is absolutely impossible that someone in the upper levels of government - and especially not a leader (like Castro) ... who is recognised by the Cuban people as an example of humility and self-discipline - could maintain personal accounts abroad."