A group of at least 20 men wielding clubs, pipes and chains attacked a crowd of 800 Cubans waiting for visas outside the U.S. diplomatic mission here today.

At least 15 persons were injured, four seriously, in fighting between the two sides. More than 400 Cubans fled into the building to escape the attackers, whom U.S. officials described as Cuban security police.

Cuban government officials later denied the incident was organized by the government, saying it was spontaneous and provoked by those waiting outside the embassy, who they said yelled at passers-by and insulted the government.

The six U.S. Marine guards in the building tried at first to stop the crowd from entering the mission, but panic broke out inside as an inner door broke and set off an alarm system locking the stairways.

[In Washington, U.S. State Department spokesman Tom Reston said the Cuban government's role in the incident was reprehensible. Reston said the demonstration against Cubans who wished to leave for the United States was "clearly permitted if not sponsored by the Cuban government."]

Wayne Smith, head of the U.S. Interests Section here, immediately filed a strong protest with the Cuban Foreign Ministry and asked that the crowds outside be dispersed.

Those inside the mission refused to leave the building, even after uniformed Cuban police had cordoned off the area. A crowd of several hundred government supporters quickly gathered outside and shouted insults at those within.

The incident was the first large-scale violent reaction to the flight of thousands of Cubans to the United States since the government two weeks ago announced that all who wish to leave the island could go. It also came on the day after President Fidel Castro denounced "U.S. aggression" against Cuba to an estimated 1 million people gathered for May Day celebrations in Havana's Revolution Square.

Castro called for an increased military alert, announced the formation of new regional defense militias and told Cubans to prepare for a "popular war" and the possibility of an "American naval blockade proposed by Ronald Reagan."

The refugee and defense issues have become increasingly linked here, with the permitted emigration of Cubans coinciding with scheduled American military maneuvers off Guantanamo Naval Base, which is operated by the United States on Cuba's southeastern tip.

On Wednesday, Washington announced cancellation of the part of the exercise, to begin May 8, calling for a 5,000-troop amphibious assault on Guantanamo, citing "humanitarian" reasons and the necessity to facilitate the cross-Caribbean flight of the refugees to Florida.

Still, Castro announced yesterday that on May 17, 5 million people -- half of Cuba's population -- will be mobilized all over the country to renew protests against the Guantanamo base, U.S. surveillance flights over Cuba and the U.S. economic blockade of the island.

Today's apparently premeditated attack seemed a clear warning to the United States not only to quickly accept the refugees, but also to downgrade military action in the Caribbean.

It began shortly after 10 a.m. Smith, the head of the interested section, had earlier gone outside to talk to a crowd of visa applicants that was four times as large as usual today. Most of those waiting, according to U.S. officials, were former political prisoners who have waited more than a year for promised U.S. visas and are becoming desperate as they see hundreds of people leaving Cuba on boats that Cuban officials will not allow the former prisoners to take.

Smith told the crowd that their visas would be processed as soon as possible, and then returned inside the building. Suddenly, according to U.S. officials and an American reporter who witnessed the events, at least one bus marked The Cuban Institute for Friendship Among People pulled up outside.

"It was a gruesome scene," said one American who witnessed it. "There was nothing casual about it. They were not provoked. They went straight for the crowd and started bashing them with clubs and pipes.

One used a plastic-covered bicycle chain."

People fled in all directions -- many of them into the building, a large waterfront structure which served as the U.S. Embassy here before the two countries broke relations in 1960, and which now houses U.S. diplomats under the Swiss flag.

Others who could not get inside the building ripped up bricks from a small wall around the lawn and began to throw them at the growing progovernment crowd, which began to hurl stones. A large window pane at the entrance to the building was shattered and more people poured inside through the hole.

Inside the mission, the lobby and waiting room were crowded with frightened people, and the floor stained with blood. Some of the Cubans had head wounds, and the more seriously hurt of the 15 wounded had been taken into a back room. A U.S. official said one man had several broken ribs and suffered from internal hemorrahging.

"He was told he would die if he did not go to the hospital," the U.S. official said, "but he refused to go. He said he'd rather die inside." u

One man who said he was a former political prisoner showed a small machete and a piece of the shirt he said he had ripped off one of the attackers. Inside the shirt pocket was an identification card which said: "Emilio Menendez. No. £00030. Ministry of the Interior, Division of Immigration and Foreigners."

"These are the types of people they let loose on us," the ex-prisoner said.

Another Cuban with a head wound said he would not leave the building "for anything in the world. Either I'll die here or from here I go to the United States."

Smith, who negotiated with high Cuban officials from the interior and foreign ministries outside the building, told the nervous refugees the Cuban authorities had offered safe passage home for everyone. By 3 p.m., 52 people had accepted and were taken off by bus. U.S. officials eventually counted 390 who remained inside at nightfall refusing to leave. The officials said they had enough C-rations inside the building to give everyone one meal.

Outside, more than 30 police cars had formed a barricade around the building and prevented crowds from apporaching. Police had arrived on the scene about 20 minutes after the attack began but, despite repeated requests from American officials, they did not disperse the crowds. A heavy rainstorm at the end of the day eventually drove most of those who had gathered outside away from the building.

Once the police arrived on the scene this morning, they were joined by ambulances and eventually one of the more seriously wounded inside was brought out on a stretcher and taken to a hospital.

Gen. Jose Abrantes, vice minister of the interior incharge of internal security, said that the incident began this morning as the visa applicants had shouted counterrevolutionary slogans at people passing in a bus.

"The people got angry," he said. "You know people's feelings here. Then a fight started and people started throwing stones." Abrantes denied there had been a premeditated attack by armed assailants."People are emotional and angry," he said. "But now everything is under control."

In front of the building a Cuban television crew interviewed two pro-Castro men who had been wounded by bricks, one with a head wound and the other bleeding from the knee.

But security police confiscated the film of an NBC crew that had filmed the fights. The police escorted American reporters away from the scene as the angry crowds started yelling threats at them.