The man whose arrest 20 years ago marked for many the real turning point in the course of the Cuban revolution today says that Fidel Castro "has never been a real communist," but is rather an "opportunist" who saw communism as his only guarantee of lifelong personal power.

Castro "managed the revolutionary process as if it were a private business; he realized that if it went democratic, he could not always be El Hombre . And that is the point where we clashed."

It is not a conclusion that Huber Matos arrived at lightly. A hero in Castro's revolutionary army and its first governor of the east-central province of Camaguey, he spent two decades in Cuban prisons following his 1959 arrest on charges of "implying the revolution was communist." The charges were later changed to treason, "trying to analyze what happened in Cuba."

For much of the time until his sentence ended Oct. 21, Matos was held incommunicado, often naked, beaten and for years kept in what he described as "concrete boxes, where the windows were covered with metal," and even the guards were forbidden to look at him.

To many anti-Castro Cuban exiles, Matos, 61, is a man returned from the dead, more a symbol of what they charge is the revolution's betrayal than flesh and blood, He is also a man who believes he earned Castro's undying hatred, not for his supposed crimes, but because throughout his imprisonment he "never bent" to blows and demands that he renounce his convictions.

Despite the long years when Cuba's most famous political prisoner was reported in failing health, as his family traveled the world in vain attempts to secure his early release through international pressure on Castro, Matos today is an older version of the same bearded young face whose clear, icy blue gaze, softened by a trace of a smile, stared out of newspapers at the time of his arrest.

In press conferences immediately following his release, when he was flown to his waiting family in Costa Rica, Matos appeared ill. As he described the conditions he endured in prison, he occasionally cried.

Now, in Washington as a special guest of the AFL-CIO national convention, to thank those such as Amnesty International who fought for his release, and to plead for political prisoners still in Cuba, he has put on weight. He speaks more calmly, but in an endless monologue reflecting the anxiety of a man with 20 years' worth of stored and uncommunicated thought.

In Cuba, Matos is considered a traitor by schoolchildren who know his history.

Ernesto Che Guevara, in his "Reminiscences" of the revolutionary war, gave Matos short shrift.

But Matos' appointment as a guerrilla squardon leader, and his later assignment as provincial governor, tell a different story of his value to the revolution, and raise the question of who Castro -- who personally testified for six hours at his trial, calling Matos "worse than a traitor; an ingrate" -- turned against him.

The late Herbert Matthews, a journalist who wrote a sympathetic history entitled "revolution in Cuba," gave one answer. "The cement, the quality that counted above all others," Matthews wrote, "was loyalty. Disloyalty was and still is the unforgivable sin, no matter what services were performed in earlier times. To Castro and his associates, Maj. Huber Matos could never have been loyal" once he questioned the revolution.

But the version offfered by Matos, and accepted by many others, is that his loyalty never wavered. "I did not conspire against Castro when I could have," Matos said in an interview last week, "because I considered him my brother, and one does not conspire against a brother."

When Matos talks about his days as a revolutionary fighter, his stories take on the same kind of animation as those of any 60-year-old recounting his wartime exploits without the 20 intervening years of imprisonment.

"I was a teacher, working first in rural schools and then in Manzanillo," near Cuba's southeastern tip. "On the day of Batista's coup in March 1952, I took the students out in the streets to protest . . . that same month I began to work clandestinely."

It was not until he narrowly escaped capture while transporting supplies to Castro's small band in the Sierra Maestra mountains in March 1957, however, that Matos went underground.

"I wanted to go into the Sierra, but they sent me back a message saying 'it's not men we lack, but arms,'" he said.

Soon, he took asylum in the Costa Rican Embassy in Havana and was flown to that country, where he spent a year colecting weapons. Castro sent a sympathetic message to Costa Rican president Jose Figueres, who added heavy machine guns and, on March 30, 1958, Matos and Cuban pilot Jose Luis Diaz Lanz landed a planeful of arms in a Cuban field.

"It was the first time I had met Castro," Matos said. "It was the time I saw him most content, most euphoric. He was like a boy, shooting the guns up into the air, handing them out. He said over and over again 'now, we are going to win the war.'"

The arms brought by Matos and Diaz Lanz, Matthews wrote, "permitted Fidel to form another column, under Matos, which carried out some successful raids in the plains."

His time as a guerrilla leader, Matos remembered, "was intense. We wanted to prove ourselves. That is how guerrillas are -- they go beyound hunger, walk until their feet bleed. Their physical force goes beyond all logic and it sometimes seems tht the war has become a war of crazy men."

Later, he said, "I always thought tht if I got out of prison alive I would tell my story, the important things that are part of a man's life. When one is a prisoner, if he can't walk around, can't live with his family, he can't smile and enjoy pleasures like listening to music, watching a movie -- all the things tht are so natural in the life of a free man -- if he is reduced to the existence of a plant, waiting for spring to come -- these are the things that give substance to one's existence. This and the certainty that one s defending a just cause."

After the rebels entered Havana in January 1959, and he was appointed governor in Camaguey, "everything went well in the first months." But then Matos said he and other commanders began to notice a change in the tone of propaganda that central headquarters sent to the soldiers.

"It had inclined toward Marxist-Leninist ideas," he said. "It made us alert, we talked a lot about it."

That summer, Matos said, he wrote of his concerns to Castro, and met with him at the new prime minister's suite in the Havana Hilton. "I told him of my concern of the necessity of taking whatever methods necessary so the revolution wouldn't be detoured; the need for a plan, a definition of the doctrinal bases of the revolution."

Castro, Matos said, persuaded him not to resign, assuring him the revolution was not taking the wrong path. But by October, Matos wrote Castro of his decision to "separate myself from the armed forces," in order not to be an "obstacle to the revolution" because of his differing beliefs.

The next morning, Castro sent revolutionary leader Camilo Cienfuegos, who Matos said shared his concern, to arrest him. Castro followed close behind, and personally denounced Matos in a mass raly in Camaguey.

That same afternoon, Oct. 21, 1959, pilot Diaz Lanz, who had defected to the United States the previous June, dropped thousands of anti-Castro leaflets from a plane over Havana.

Cuban fighter-bombers pursued him and, in the process, according to Matthews and other historians, dropped several small bombs on the city. Castro charged that the bombs had come from the escaping plane of Diaz Lanz. Castro said the project had been launched by the CIA and he organized the first of a series of massive anti-U.S. demonstrations.

During the revolution, Matos said last week, "I was with Castro; sometimes we spent whole nights there lying on the floor, talking. Certainly, he always kept part of himself inside, there were things not spoken about. But he knew what I thought, and I believe I knew what he thought by what he said to me.

"I don't believe he was a real comminust . . . I believe that when the revolution came to power, Fidel saw two possibilities for the furture -- a democratic revolution . . . in which he would not be the man to govern Cuba for the rest of his life . . . and communism' which Matos said was promoted by Guevara and Castro's brother, Raul.

"All the changes could have been made, all the reforms democratically," Matos maintained, "but he would have had to have taken the chance that somebody else could have taken over the country, and this clashed with the personal interests of Mr. Fidel Castro."