US and European diplomats have called for the immediate release of 75 Cuban activists jailed last year during a major crackdown on political dissent.
Rivero: Jailed for trying to exercise free speech, his family says
They made their appeal at the Havana home of Cuban poet and journalist Raul Rivero who is serving 20 years in jail.
The diplomats' action came as the UN cultural organisation awarded Mr Rivero its World Press Freedom award.
The prize was a tribute to Mr Rivero's "brave and long-standing commitment to independent reporting," Unesco said.
Diplomats from several European countries, including Spain and the UK, as well as the head of the US Interests Section, James Cason, gathered at Mr Rivero's house to offer their support for the 75 dissidents who were rounded up last year.
Many were given lengthy prison terms, accused by the Cuban authorities of collaborating with the US to try to undermine President Castro and his government.
Mr Rivero, a critically acclaimed poet and leading journalist, is one of the best-known of those jailed.
The BBC's Stephen Gibbs says that while the Cuban government has tended to be scathing about the professional credentials of many of those who describe themselves as independent reporters, it has been unable to dismiss Mr Rivero with such ease.
He worked for many years for the government press agency, before he left in 1988 and subsequently set up an illegal independent news agency and helped to train other reporters.
Before his arrest last year, he wrote occasionally for American publications - evidence, according to Cuban officials, that he was in the pay of an enemy country and deserved 20 years behind bars.
Mr Rivero denied the charges.
At a ceremony in the Serbian capital Belgrade, Unesco director-general Koichiro Matsuura praised Mr Rivero's brave commitment to independent journalism and called for his release.
Mr Rivero's wife, Blanca Reyes, said she was denied permission to leave Cuba to collect the award.
Instead, her son Miguel Angel Sanchez Reyes, who lives abroad, was their representative.
"The poet and journalist Raul Rivero is being held prisoner in a jail over 450 kilometres from the capital of Cuba for wanting to exercise the right to free speech under a regime where the word right, if not freedom, simply does not exist," he told the Unesco ceremony.