The cause was a heart attack, his wife, Flor Ester Torres Sanabria, told The Associated Press.

On Dec. 8, 1934, Mr. Gutiérrez-Menoyo was born into a Spanish family dedicated to fighting for freedom: his oldest brother died fighting fascists in Spain's civil war. The family moved from Madrid to Cuba in 1945. In 1957, another brother died in a failed attack against the Cuban strongman Fulgencio Batista. Eloy soon joined the fight against Batista, assembling an army in Cuba's mountains almost as large as Mr. Castro's own force.

After Batista fled on Jan. 1, 1959, Mr. Gutiérrez-Menoyo and his troops arrived in Havana on Jan. 3, days before Mr. Castro led his own army into the city. Mr. Gutiérrez-Menoyo was not offered a post in the Castro government, and later said he had not wanted one. He retained the rank of major, the highest in Cuba at the time. His army was absorbed into Mr. Castro's.

But disturbed with Mr. Castro's turn toward leftist dictatorship, Mr. Gutiérrez-Menoyo and a dozen military and civilian supporters fled by boat to the United States in January 1961. He settled in Miami and formed an organization to overthrow Mr. Castro, naming it Alpha 66. It was not involved in the failed Bay of Pigs invasion in April 1961, and it grew to become one of the largest anti-Castro exile groups.

In October 1964, Mr. Gutiérrez-Menoyo led a cadre of men into the Cuban mountains to establish a secure base from which to expand to fight the Communist state, just as Mr. Castro had done to fight the Batista government. He told interviewers that the mission was financed by other exiles, not by the Central Intelligence Agency.

Mr. Gutiérrez-Menoyo and his men were captured by government troops. Blindfolded, he was taken on a 90-minute plane ride, and when the blindfold was removed he found himself facing Mr. Castro, who was sitting behind a desk surrounded by his top aides.

"You realize, of course, that we are going to shoot you," he quoted Mr. Castro as saying in a 1987 account in The New York Times Magazine.

Mr. Gutiérrez-Menoyo told Mr. Castro of his family's revolutionary past and said he would accept death. Mr. Castro said he would also kill his men but would spare them if Mr. Gutiérrez-Menoyo would go on television and affirm that no Cubans wanted to overthrow their government. He accepted the deal and later, after a 30-minute trial, was sentenced to 30 years in prison.

As an inmate, Mr. Gutiérrez-Menoyo refused to work or wear a prison uniform, saying the requirements violated international law covering the treatment of political prisoners. For his refusal he was beaten so badly that he became blind in one eye and deaf in one ear. He spent time in six prisons over 22 years before being released in 1986, largely through the efforts of Prime Minister Felipe González of Spain.

Mr. Gutiérrez-Menoyo returned to Miami in 1993 and set up an organization to encourage peaceful dialogue between the Cuban government and antigovernment exiles. He called it Cambio Cubano (Cuban Change).

He said he was determined not to be embittered.

"When you spend entire years in solitary, you learn that human beings have a lot of inner resources," he told The Times in an interview in 1993. "When you are subjected to a policy of savagery and barbarism, you come to the conclusion that you have to reject those methods, that you have to be the first to set hatred aside, otherwise it will destroy you."

In 1995, Mr. Castro agreed to Mr. Gutiérrez-Menoyo's repeated requests for a meeting. They had a three-hour exchange in Havana, during which Mr. Gutiérrez-Menoyo pressed Mr. Castro to allow Cambio Cubano to function as an opposition party in Cuba.

That never happened, but Mr. Gutiérrez-Menoyo was allowed to return to Cuba repeatedly and, in 2003, to settle there. He did not succeed in opening an office for his organization, but he did meet with Cuban moderates.

"Cuba cannot continue to corner itself, trying to convince the world that there is democracy, when a one-party system will never be a democracy," he said.

In addition to his wife, Mr. Gutiérrez-Menoyo is survived by three sons and a daughter, The A.P. said.

When they met in 1995, he and Mr. Castro had not seen each other since Mr. Castro had threatened to shoot him three decades earlier. Mr. Castro's first words to him, he recalled, were, "You've let your hair grow long."