Three anti-Castro Cuban exiles were convicted yesterday on all counts in connection with the 1976 bombing assassination here of former Chilean Ambassador Orlando Letelier and a colleague.

The U.S. District Court jury of seven women and five men deliberated for 8 1/2 hours over the last two days and then told Judge Barrington D. Parker in a hushed courtroom that it had found the defendants -- Guillermo Novo Sampol, his brother, Ignacio Novo Sampol, and Alvin Ross Diaz -- guilty on all 15 charges that they faced stemming from the Embassy Row bombing death.

The killing of Letelier, one of the most ardent critics of the current Chilean military dictatorship of Gen. Augusto Pinochet, was the most notorious act of international terrorism ever committed here. Indeed, the jury accepted the government's contention that the Chilean secret police, once known as DINA, had masterminded the murder plot.

Relatives and friends of the three defendants sobbed as the jury foreman read the guilty verdicts. The three defendants, surrounded by six U.S. marshals and protected by an additional 16 marshals in the courtroom, stood stony-faced as the verdicts were announced.

As they were led to the cellblock behind the courtroom, Ignacio Novo and Ross raised their fists and shouted: "Viva Cuba!"

The outcome of the case was presaged moments before the jury returned to the courtroom. Guillermo Novo looked at his friends in the courtroom and said in Spanish, "It's sure that they screwed us."

As he made the comment, Novo drew a hand across his throat.

In rendering the guilty verdict, the jury accepted the contention of three young U.S. prosecutors -- Eugene M. Propper, E. Lawrence Barcella Jr. and Dianne H. Kelly -- that Letelier's slaying was carried out by the Cubans in conjunction with the government's key witness, Michael Vernon Townley.

A 36-year-old American-born DINA agent who refers to the Chilean secret police as "my service," Townley told the jury that he recruited the Cubans to help in the assassination mission on orders of his DINA superiors.

Townley pleaded guilty in August to murdering a foreign official in exchange for a government promise that it would recommend parole for him after he serve the minimum amount of 3 1/3-to-10-year sentence.

Townley, who has about 2 1/2 years to go on the terms, is not required by his plea bargain agreement to tell U.S. officials about any terrorist activities he might have committeed that do not involve U.S. citizens or were not committed in the United States.

Immediately after verdict, Propper predicted that the decision "should make it easier for us" to win the extradition of former DINA chief Juan Manuel Contreras Sepulveda, DINA operations director Pedro Espinoza Bravo and DINA agent Armando Fernandez Larios, all of whom are charged with the Letelier slaying.

The three former DINA officials are awaiting a decision by the Chilean Supreme Court on whether they must be extradited to stand trial in the U.S.

Although only the Novo brothers and Ross officially were on trial, many people in Chile and the U.S. familiar with the case have viewed the ultimate defendant as the government of Pinochet, who led a junta that overthrew Marxist President Salvador Allende in a bloody coup in September 1973.

The Pinochet government imprisoned Letelier and other Allende officials and supporters without placing charges against them. Eventually, Letelier was expelled from Chile. After he moved to the United States, he became the most vocal critic of Pinochet and the alleged human rights violations his government had committed.

The 44-year-old Letelier was stripped of his citizenship just days before he and a colleague of his at the Institute for Policy Studies, Ronni K. Moffitt, were killed as they rounded Sheraton Circle on their way to work.

Many U.S. and Chilean officials have said privately that they do not believe the three Chileans will be extradited, partly because Contreras is a close ally of Pinochet. Two other Cuban exiles charged with the Letelier and Moffitt slayings, Jose Suarez Dionisio Esquivel and Virgilio Paz Romero, remain fugitives despite a worldwide search.

Guillermo Novo and Ross both face mandatory 20-year prison terms for their conviction of the first-degree, premeditated murder of Letelier and could receive up to a life sentence. Novo and Ross were both convicted of conspiracy to murder a foreign official, murder of a foreign official, and first degree murders of both Letelier and Moffitt, and murder by use of explosives. In addition, Novo was convicted of two counts of lying to the grand jury that investigated the Letelier killing.

Ignacio Novo faces up to 13 years in prison for lying to the grand jury and for failing to tell authorities about the crime after he learned of it from Townley.

Parker set no date for sentencing, but told defense attorneys to make any post-trial motions by March 5. Parker sent all three Cubans to jail without bond pending sentencing.

Defense lawyers Paul Goldberger, Lawrence Dubin and Oscar Suarez immediately said they would appeal the verdicts.

"Accepting Townley is hard for anyone to do," Goldberger said, "but I guess that's what the jury did."

The 22-month investigation that led to the indictment of the five Cubans and three Chileans last August was headed by Propper and a drawling FBI agent named L. Carter Cornick.

As the prosecution team basked in its victory yesterday, the 31-year old Propper chided Cornick, saying "Hey, Carter, weren't you the one that said this case would never go to trial? You owe me a dinner."

U.S. Attorney Earl J Silbert, who personally notified U.S. Attorney General Griffin B. Bell of the verdict, said that the decision "was a just one, a fair one."

Silbert said the verdict shows that "terrorism anywhere, but particularly in the nation's capital, will not be tolerated. He pledged that the government, "to the best of our efforts," would persuade the extradiction of the Chileans and the arrest of the two fugitive Cubans.

The trial of the three Cubans brought the drama of clandestine international intrigue to a Washington courtroom. At various times during 21 days of testimony and arguments, some of it heard with the jury outside the courtroom, there were tales of a midnight rendezvous to plant the bomb in Letelier's car, an abortive assassination mission to Mexico, and Ross' discussion of an alleged plot to blow up Russian ships in American harbors and his purported participation in three assassination attempts on the life of Cuban Premier Fidel Castro.

The Letelier slaying generated extraordinary passions 2 1/2 years ago that still were much in evidence in and around Parker's sixth-floor courtroom in the federal courthouse, a short distance from the Capitol. When Townley first appeared in the courtroom last month, the Cuban defendants taunted him with a course of invective in Spanish. Relatives and friends of the defendants and Letelier watched the trial each day.

Because of the nature of the case, threats that had been made against the lives of Parker and Propper and the fact that two Cubans still are fugitives in the case, wary courthouse officials imposed unprecedented security.

German shepherds trained to sniff for bombs searched the courthouse daily for explosives, but found none. Visitors to the courtroom had to pass throught two magnetometers to get inside and leave some form of identification with U.S. marshals

As it turned out, the only fireworks were inside the courtroom and they were verbal. Prosecutors and defense attorneys sparred daily over the admissibility of evidence, so much so that the jury on several days heard less than two hours of testimony.

From virtually the beginning of the testimony on Jan. 15 to the closing arguments earlier this week, the case seemed to hinge on Townley's testimony.

Through 3 1/2 hours of direct questioning by prosecutor Propper and five days of grueling cross-examination by defense attorneys, Townley matter of factly told the jury in an often-packed courtroom of a part of his life as an international terrorist. Judge Parker ruled, however, that defense lawyers could not ask Townley about his alleged participation in trips to Italy and Argentina to kill Chilean exiles.

A native of Waterloo, Iowa, who now calls Chile his homeland, Townley testified that although he regrets the slaying of Moffitt, he has no remorse about killing Letelier.

"He was a soldier, I was a soldier," Townley said calmly, in much the same manner someone might say he doesn't care what's for dinner.

Townley, a self-taught electronics expert who has spent more than 20 years in Chile, said he started his life as a terrorist with surreptitious hit-and-run activities against the Allende government, eventually became an informant for DINA and later an agent.

Acting on orders of his DINA superiors, Townley said he traveled to Mexico in early 1975 in an effort ot "eliminate" two Chilean exiles, but that he and his wife Mariana, another DINA agent, arrived a day after a meeting of exiles had ended.

By mid-1976, Townley testified, his DINA superiors had decided to kill Letelier, who increasingly had become an outspoken critic of the Pinochet government for its alleged human rights violations.

Defense lawyers argued that Townley was much more than a DINA agent contending that he killed Letelier on orders of the Central Intelligence Agency. Neither testimony nor any form of evidence ever supported the contention, but the defense lawyers pointed to Townley's four contacts with the CIA in the early 1970s as a suggestion that the CIA could have been involved in the killing.

For the husky, broad-shouldered Townley, killing Letelier was just another order to carry out.

He said he initially was thwarted in gaining entry to the United States when false Paraguayan passports he had obtained were revoked. But in early September 1976, Townley said, he entered the U.S. under the alias Hans Peterson Silva and traveled to the New York City area to recruit members of the Cuban Nationalist Movement to help him carry out the assassination.

Although the Cubans said they were angry about the Chilean government's expulsion of Cuban exile Rolando Otero to the U.S., Townley testified that the Cubans eventually agreed after a strategy session at a New Jersey motel attended by Guillermo Novo and Ross to participate in the killing. One of the Cuban nationalist leaders approving the Letelier mission was Novo, Townley said.

Townley said he assembled the bomb in a Washington hotel with the assistance of fugitives Paz and Suarez and then drove to Letelier's Bethesda home in the early hours of Sept. 19, 1976. There, Townley attached what he antiseptically called "the device" to the undercarriage of the former diplomat's Chevelle.

The bomb was detonated by remote control as Letelier and Moffitt drove to work two days later. Moffitt's husband of 3 1/2 months, Michael, was a passenger in the back seat, but was injured only slightly.

In a final bizarre episode in the trial, the defense unsuccessfully tried to get Parker to let the jury hear about a telephone call Townley made Jan. 30 to a former DINA agent in Chile in which Townley allegedly suggested that their friends call Parker and threaten him to get him to withdraw from the case.

The defense otherwise focused its attention on trying to discredit the testimony of three convicts, all of whom implicated Ross and Guillermo Novo through statements they said the defendants made after the assassination. The defense also tried to chip away at details in the prosecution's case in an effort aimed at trying to create a "reasonable doubt" in the jurors' minds that the defendants were guilty since court rules require the prosecution to prove a case "beyond a reasonable doubt."