Anastasio Somoza Debayle, the exiled former president of Nicaragua, was assassinated by gunmen this morning on a street near his home in Asuncion, Paraguay.

According to reports reaching Argentina from Asuncion, Somoza died in a barrage of machine-gun fire as he rode down an Asuncion street in his white Mercedes-Benz. The reports said a pickup truck with three men in it intercepted the car behind Somoza, which carried his bodyguards. At that moment, the reports said, someone began shooting from inside a nearby house that had recently been rented, apparently to Somoza's assassins.

The 54-year-old former president was killed immediately, as were his chauffeur, Cesar Gallardo, and economic adviser Joseph Bertnier. Paraguyan police said they had counted 25 bullet holes in Somoza's body. They said that what appeared to be a bazooka blast from the gunmen's house had nearly ripped off the roof of the car.

"The top of it is completely destroyed," said one person who saw the scene after the killing. "You can see bullet holes in the body of the car."

The three men in the pickup truck drove away immediately after the shooting, according to reports. Three other men ran out of the rented house and accosted a passing Argentine man in his car. Speaking with the distinctive accent of Argentine or Uruguayan Spanish, the men cried, "Get out," the motorist was quoted as saying.

Police in Paraguay were quoted as saying late today that two suspected attackers belonged to an Argentine guerrilla group called the People's Revolutionary Army. Photographs of the two suspects, both Argentines, were distributed, but there were no immediate arrests, according to reports from Paraguay.

A Paraguayan Interior Ministry communique said it was nearly certain that "a foreign extremist group" assassinated Somoza. The statement said Paraguayan authorities were carrying out an "intense search" for the attackers.

The pickup truck was found abandoned about three blocks from the site of the shooting. According to reports, it still contained some blue overalls, sausages and bread and copies of a comic book popular in both Paraguay and Argentina. Inside the house, which was empty of furniture, a walkie-talkie and a bazooka reportedly lay on the bare floors.

The mustachioed and portly Somoza was the last ruler in a 44-year dynasty in Nicaragua that was finally toppled after a protracted civil war. He has lived in Asuncion since August 1979, one month after he fled his homeland. Under the official protection of Gen. Alfredo Stoessner, the passionately anticommunist Paraguayan president whose hold over his country has been the strongest and longest in all of South America, Somoza acquired a spacious suburban home, a swimming pool, some Paraguayan investment property and a colorful social reputation.

For example, Somoza is said to have infuriated Stroessner's former son-in-law by temporarily stealing his mistress. In the initial confusion following this morning's assassination, therefore, some members of the diplomatic community suggested that the killing might have been personally rather than politically inspired.

After the shooting, telephone lines to Paraguay were immediately jammed with calls, and all flights to Asuncion were temporarily canceled. This made information on possible motives difficult to acquire. But there was no question that in the course of his life, Anastasio Somoza Debayle made a great many enemies.

They included many Nicaraguans, from those who were involved with him in unfortunate business dealings and members of the Sandinista National Liberation Front that overthrew him to former National Guard officers and officials in his government who felt that his resignation under pressure betrayed them.

Although two Nicaraguan diplomats were expelled from Paraguay without explanation last month, Western diplomatic sources said the expulsions had nothing to do with politics, but were related to involvement in Paraguay's substantial contraband trade.

The glossy Argentine magazine, Gente, recently printed an interview in which Somoza vigorously defended his leadership of Nicaragua and maintained, as he did through the most critical months of civil war, that communists and terrorists had taken control of his country.

"You know, the initials of the Sandinista front are FSLN" for Frente Sandinista de Liberacion Nacional , Somoza said. "Now the people are painting the same message, but they put right next to it, "Favor Somoza Liberar Nicaragua " (please, let Somoza liberate Nicaragua).

In the interview Somoza estimated his wealth at approximately $20 million when he entered Paraguay. "And it was a hundred million before I lost what I lost in Nicaragua," he said. "Although I haven't lost it, I'll get it back. Because no one has the right to take from a person what he has worked for honorably."

In Nicaragua, meanwhile, government officials of the revolutionary movement that ousted Somoza said the attack was carried out by Paraguayan "freedom fighters," but gave no source for their information.

A member of the ruling Nicaraguan junta, Rafael Corboba Rivas, said a trial of Somoza in absentia had been under way in Nicaragua. He said Somoza specifically was charged in the assassination of publisher Pedro Joaquin Chamorro in 1978.

His wife, Violeta Chamorro, said after learning of the assassination, "I knew God had to do justice some day and Somoza's hour has come."

Chamorro's joy was widely shared in Managua, according to reports from that city, and residents embraced each other and danced in the streets after hearing the news. The Sandinista government declared a "national day of celebration."

The Asuncion radio station Primer de Marza said the attack occurred at about 10 a.m. at an intersection seven blocks from Somoza's villa and two blocks from the U.S. Embassy. Somoza reportedly was on his way downtown, either to go shopping or perform his daily exercises.

His Mercedes was followed by another car carrying a driver and two bodyguards, reports said.

According to one account, the attack started with small-arms fire from a blue Chevrolet pickup truck. Then machine-gun fire burst from the near-by house, and one of two bazooka rockets hit the Mercedes.

After the attack, police reportedly had to take the wrecked car to a laboratory to extract Somoza's bullet-riddled body.