The US State Department called for the release Tomas Nunez Magdariaga, a member of the Patriotic Union of Cuba (UNPACU), one of the island's largest dissident groups.
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He had been convicted on "false charges and convicted...in a sham trial", it said.
UNPACU leader Jose Daniel Ferrer says Nunez was sentenced to a year in jail for allegedly making threats to a security agent--a charge the agent later recanted, according to Ferrer.
"We understand his health is in a critical state, and that the authorities have denied his family the opportunity to see him," the State Department said in a statement.
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The Cuban government has not commented publicly on the case and did not immediately reply to a request for comment. Havana considers all dissidents to be agents of the United States. It regularly detains them for a few hours or days, as part of what critics and human rights groups describe as a long-running campaign of harassment and repression.
In August, authorities accused Ferrer of attempting to run over a government official and arrested him but released him after 12 days.
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