The affair started with a little-noticed report that five Cubans were condemned to death for trying to start a Solidarity-style independent union.

Three months and many accusations later, it is still unclear precisely what the five Cubans did or what happened to them afterward. But in the meantime, President Fidel Castro's government has been pushed into a startling admission that a major antigovernment sabotage ring was burning fields and shops here. And the United States, in an effort to publicize human rights aspects of the case, has suffered a stinging demonstration of its isolation in the Reagan administration's confrontation with Cuba.

The first reports said Latin American labor activists were seeking help from Costa Rica to persuade Castro to spare the five. Without detail and fuzzily sourced, they were given little public attention. But to the U.S. government, the human rights issue presented a good opportunity to portray Castro's rule in a bad light.

The chief of the U.S. interests section in Havana, John Ferch, called ambassadors from a dozen U.S. allies to a meeting at noon in the former U.S. Embassy that rises prominently on a square just off Malecon boulevard along the Havana seaside. Some of the European ambassadors thought they were about to be briefed on overflights by U.S. SR71 spy planes, which Cuba had recently complained about.

But when they got there, and when their diplomatic cars were parked visibly in the square, Ferch said he was seeking their advice and association for a diplomatic demarche to the Cuban government over the reports.

No one agreed to join in, according to participants, and no one thought it was a good idea for the United States to go ahead alone. One European ambassador reportedly walked out before the meeting was over, eager to dissociate his government from the whole idea. Others complained that Cuban officials might conclude from the cars gathered around the U.S. building that European allies already were part of the American plan.

"Join with the United States for a demarche here?" said a European diplomat. "It was madness, a stupidity."

Ferch nevertheless made representations to Deputy Foreign Minister Ricardo Alarcon in the third week of April. No other country joined, diplomats here reported, although Italy was said to have brought the subject up in Rome and French Ambassador Pierre Descamps warned Cuban officials that journalists in town with a visiting French minister would probably be asking about it.

According to European diplomats, the incident underlined the U.S. isolation here in defending administration policies on Cuba.

At that point, said a well-informed diplomat, the most reliable reports indicated a labor disturbance broke out Jan. 25 at a sugar factory in Artemisa, 30 miles southeast of Havana, after a truck driver was dismissed for what his fellow workers considered unfair reasons.

A ruckus and perhaps a factory occupation ensued, he added, and somebody may have shouted about a new union during a confrontation with police. There were reports of death sentences but the Cuban government denied them and no one claimed to know for sure.

About a week after Ferch saw Alarcon, the original charges were picked up and amplified by the International Confederation of Free Trade Unions in Brussels. The group, the major anticommunist labor federation, said the five death sentences had been commuted to 30 years' imprisonment, but added that four Cuban lawyers were arrested for defending the suspects too vigorously.

This apparently prompted the Cuban government to reply. Vice President Carlos Rafael Rodriquez told visiting French reporters that the five had never been condemned to death nor had they tried to form a new union, and that the lawyers were arrested in a separate case of bribing a judge.

More revealingly, Roberto Veiga, secretary general of the Cuban Workers' Central, wrote to the Communist-oriented World Federation of Trade Unions in Prague that the real number of those jailed was 33 and "other persons" also had been arrested. The 33 were convicted of "numerous and continuous acts of sabotage" and "preparation for large-scale acts of sabotage" and "preparation of assassination attempts against the country's leaders," he added.

The sabotage included burning crops and work centers and attacking highway traffic, the May 13 letter said. Plots also were under preparation to extend the counterrevolutionary activities to setting fires in Havana, it added. "Thus there is not the slightest case of violating human rights, and even less a case of violating union rights," Veiga said.

"There is a group of persons who under the inspiration of the Cuban counterrevolution living in the United States and with the support of the CIA and the stimulation of the Reagan government seek to interrupt the peaceful construction of socialism and interfere, endangering citizens, with the normal producation activities of our country and assassinate Cuban leaders approved not only by popular vote but also by the daily consensus of our people."

Diplomats in Havana called the letter an extraordinary admission that counterrevolutionary sabotage continues in the Cuban countryside more than 24 years after the revolution, perhaps more significant than the original reports of independent union agitation.

The accusations contained nothing to indicate the sabotage had been carried out by anti-Castro Cubans coming from Florida, as has been the case in some past incidents. Although the Cuban government frequently complains of counterrevolutionaries, it normally seeks to portray Cubans on the island as united behind Castro's leadership. But the letter, telexed to local news agency offices, indicated a serious campaign.

Although it did not detail all 33 of those convicted, it said the five spoken of abroad included four black-market dealers and a small farmer.