Eloy Gutierrez Menoyo, a former Cuban revolutionary who later became a dissident, has died aged 77, friends and family have said.
His wife, Flor Ester Torres Sanabria, told AP news agency that her husband died in a Havana hospital after suffering a heart attack.
Mr Gutierrez Menoyo was a commander during the 1959 Cuban revolution.
However, he later led an armed uprising against former comrade Fidel Castro and spent 22 years in prison.
A close friend in Cuba, radio commentator Max Lesnik, said the opposition activist had died on Friday morning, the Miami Herald reported
Born in Madrid, Mr Gutierrez Menoyo was the son and brother of men who fought in the Spanish civil war against Gen Francisco Franco.
The family moved to Cuba in 1945 and Mr Gutierrez Menoyo later joined rebels opposing dictator Fulgencio Batista.
However, after the revolution he lost faith in the communist leadership and by 1961 was in exile in Miami helping to form Alpha 66, an armed commando group.
Mr Gutierrez Menoyo and his fighters returned to Cuba in December 1964 hoping to launch an uprising, but they were captured after a month.
He then spent 22 years in Cuban prisons until being freed through a petition of the Spanish government in 1986.
He later moved back to Miami where he founded Cambio Cubano - a centrist group that promoted dialogue and reconciliation among Cubans of all political backgrounds.
Mr Gutierrez Menoyo returned to Cuba in 2003. The authorities allowed him to stay despite his frequent criticisms of the government.
In 2008 he expressed his disappointment that Cuba's communist system had remained unchanged.
"Cuba cannot continue to corner itself, trying to convince the world that there is democracy here when a one-party system will never be a democracy," he said.