"Fidel Castro may be one of the world's most admired and most hated statesmen," said a long-time Cuba watcher recently, "but in essence he never stopped being the guerrilla fighter he was 20 years ago."

Faced with a bleak economic picture at home, strained relations with the United States and several Latin American friends, and the humiliating sight of thousands of Cubans opting to flee the country, Castro's prestige abroad may have suffered the worst setback since the great sugar harvest failure in 1970.

But in the in the past month he has reminded the world of what even his opponents describe as his inventive political genius, his mastery of diversionary tactics, and his ability to take the sting out of devastating blows, even turning some to his advantage.

Today, in what appears to be largely a reaction to Castro's unconventional diplomacy, the United States called off large-scale military maneuvers scheduled to start May 8 at Guantanamo Naval Base on Cuba's southeastern tip.

The Pentagon announced that the maneuvers, which were to include a mock amphibious assault by 5,000 troops at Guantanamo, have been cancelled for "humanitarian' reasons so that the vessels involved can provide reinforcement for the Coast Guard to "save human lives and ensure maritime safety during the unprecedented emergency caused by the flood of refugees leaving Cuba."

[A violent storm during the weekend caused havoc among the refugee boats and left at least four dead.]

Informed sources here had said earlier that Castro was considering opening a second port for the mass of refugees, this time close to Guantanamo. The maneuvers are perceived here as an affront to national pride and a potentially serious threat.

Such a new port would ostensibly serve as relief for the refugee port of Mariel, now congested by about 1,700 American small craft, and also offer an out for the dissidents in eastern Cuba who have to travel more than 700 miles to Mariel.

But above all, it would put tiny American refugee boats in counterpoint to vast American warships, not only ridiculing the U.S. military but also perhaps interfering with the maneuvers.

"Castro conducts politics in the old guerrilla-fighter style," said a knowledgeable resident here. "He responds fast, constantly changing tactics, and takes full advantage of the weakness of his main enemy, the United States."

Another example of Castro's deft maneuvering came yesterday morning as more than 700 former political prisoners -- 500 more than the daily averaged -- appeared at the U.S. interests section in Havana to press for visas. Sent by the immigration authorities, their presence was Cuba's fast response to criticism from Vice President Mondale, who has called on Castro to release political prisoners to be immediately picked up by American planes.

Castro's reply -- sending the ex-prisoners -- was a pointed reminder that more than 4,000 released prisoners ostensibly eligible for U.S. visas under the latest "parole program," but subject to processing delays in Washington, were still being ignored. Many of them have waited more than a year for U.S. visas, without jobs or food rationing cards, and are close to despair, according to sources close to the U.S. diplomats here.

Embarrassed and empty-handed, U.S. officials were forced to disperse the crowd outside the embassy with more promises. Later in the day, Washington sent sudden authorization to begin issuing the upheld visas.

Another part of the riposte to Mondale came in the official newspaper Granma, which editorialized: "Hundreds of Haitians have shipwrecked on our coast trying to emigrate to the United States." If Mondale wanted to be "human and just," he should urgently send planes to pick them up, adding tongue in cheek, "or would he rather we send them to him via [the refugee port] Mariel?"

But the message seems significant. Usually well-informed sources say that Cuban officials are seriously considering sending the more than 1,000 shipwrecked Haitians living in eastern Cuba on to the United States.

Since the refugee problem began, the Cuban press has repeatedly pointed out apparent contrast in U.S. attitudes toward Haitian and Cuban refugees. Like the Cubans, the more than 20,000 Haitian "boat people" now in southern Florida cite economic hardship and political repression as reasons for fleeing their country. But, so the official newspapers point out, while Cubans have always been warmly received, Haitians who live under a rightist dictatorship are threatened with deportation.

Cuba's current purge of dissidents comes in the context of serious economic difficulties and repeated criticism of the flagging revolutionary spirit by both Fidel Castro and his brother, Raul. Unexpectedly, Castro had begun earlier this year to say thinks like "building socialism and communism is a voluntary task." In a speech last March he hinted broadly he was considering a new wave of mass emigration.

Privately, Cuban officials have estimated that some 250,000 Cubans would leave the country if given the chance and a place to go.

How exactly Castro planned to stage his mass emigration may never be known, but he used the killing of a Cuban policeman at the Peruvian Embassy to unleash the refugeees. It has become common wisdom that Castro miscalculated and had not expected the embarrassing sight of more than 10,000 people storming the place. True or not, the fact is Castro nonetheless turned the onslaught of refugees and the ensuing international pity for them into the trigger he needed for the purge he apparently had planned.

Having decided on the need for a purge, Castro is trying hard to put the best possible face on the effort to consolidate his political base at home. Westerners, although very critical of Castro, already point out that no other communist regime, with the possible exception of Yugoslavia, has been so flexible in its emigration policy.

U.S. diplomats here privately noted that Castro has craftily capitalized on the angry and at times contradictory statements and threats from Washington about the flood of Cubans approaching Miami.

The threats from Washington against Miami boat owners bringing in "illegal aliens" to the United States every day have been given almost triumphant display in the press and are a major topic among pro- and anti-Castroites. "Castro has not only put the United States on the defensive," said a Western diplomat here, "he has managed to turn domestic unrest into a problem first between Cuba and the United States and now between Miami and Washington."

Yet the leadership here is clearly aware of the damage the burst of refugees has done to Castro in Latin America.

Not only have relations with Peru, Venezuela, Colombia and Costa Rica become strained over various refugees squabbles, but the continent's leftist movement will inevitably suffer from the discrediting of their hero, Fidel Castro.

Privately, Castro told a Latin American official recently that if necessary he could live without good relations with many of the governments of the continent. "We have had to live without them for many years, we're used to that," Castro was reported as having said. In the midst of the squabbles with the Latins, however, Castro could play an unexpected card.

Mexico refused to take on Peru's interest when Peru wished to break relations two weeks ago after the refugees poured into its Havana embassy.

In addition, Castro was able to give broad display to the news that the President Jose Lopez Portillo of Mexico would visit Cuba July 31. The arrival here of Gustavo Carvajal, leader of the Mexican government party, was heralded in Cuba. Unlike other Latin American countries, the Mexicans have never broken relations with Cuba.

The question now is whether Castro can use the refugee exodus to consolidate his support at home. Even if the government believes it has a quarter of a million firm opponents -- out of a population of 10 million -- many Castro supporters also are believed to be tired of the shortages of material goods, the drudgery of life under communism and the claustrophobia that comes with close scutiny of one's private life.

The government has tried to whip up public hatred against those leaving and in many instances it has succeeded, judging by the number of hecklers ready to harass refugees.

But there are indications that many moderate Castroites are deeply indignant about the way Cubans were turned against each other and how violence against those leaving was officially tolerated.

High officials here say they believe Cuba has serious economical difficulties, but no deep political problems. Fidel Castro reportedly told a visitor recently that refugee opposition to the regime would serve to inspire the young people who have never had to defend the revolution and may now find a role for themselves.

"This may be true," one veteran observer here said, "but Castro has taken the political lid off and a lot of people are blowing off steam. The question is: does he want to keep it off or can he put it back on if he wants to. If so, at what political price?"