Defense lawyers for three Cuban exiles on trial in connection with the 1976 assassination here of former Chilean ambassador Orlando Letelier accused the Central Intelligence Agency yesterday of masterminding the killing. But a prosecutor contended that the Chilean secret police had ordered the slaying.

"This is not a pipe dream," defense lawyer Paul Goldberger told a U.S. District Court jury of seven women and five men in his opening statement. "As hard as it is to believe, the CIA was responsible for a murder in the nationhs capital."

Goldberger asserted that the defense will prove that the government's key witness, Michael Vernon Townley, an American-born Chilean secret police agent who has confessed to planting a bomb that blew up Letelier's car, was a double agent working for the CIA as well as the Chilean secret police, once known as DINA.

Earlier, Assistant U.S. Attorney Eugene M. Propper, who has investigated the slaying since the day it occurred, accused DINA of directing the assassination. He said the slaying was the ultimate retribution for the strident criticism Letelier was leveling at the time against the military dictatorship of Gen. Augusto Pinochet.

Goldberger offered the jury a contrasting view.

"This was a monstrous crime committed by monstrous people," Goldberger said, "but we will prove that neither DINA, nor the Cuban Nationalist Movement nor the Chilean government had any responsibility for it." The three staunchly anti-Castro Cuban exiles on trial -- Guillermo Novo Sampol; his brother, Ignacio Novo Sampol, and Alvin Ross Diaz -- are all leaders of the Cuban group.

In claiming CIA involvement in the Sept. 21, 1976, Embassy Row assassination of Letelier, Goldberger cited previously disclosed CIA contacts with Townley in the early 1970s and the fact that one arm of the agency once sought approval to use Townley "in an operational capacity." But Goldberger cited no new evidence of CIA connections with Townley.

Propper said the government will prove that the Cuban Nationalist Movement, a militant anti-Castro group based in New Jersey, had a continuing relationship with Chilean government officials and DINA since early 1975.

In one example of that relationship, Propper said, Guillermo Novo supplied explosives and detonating devices to Townley, who had been ordered by DINA to go to Mexico to kill Chilean exiles opposing the Pinochet regime. In exchange, Townley had promised that any leftover supplies would be given to the Cuban group. But the exiles' meetings on Chile had ended by the time Townley got to Mexico, Propper said.

In the end, the prosecutor charged, the Cuban exiles only agreed to help Townley kill Letelier as long as Townley participated in the assassination. The Cubans required his participation as a DINA agent, Propper said, because the Cuban group's leaders were still angry at the Chilean government's earlier expulsion to the United States of a Cuban exile, Rolando Otero. Otero was later convicted of a bombing at Miami International Airport.

Letelier served in the Chilean government of Marxist President Salvador Allende, who was overthrown in a 1973 coupe by the junta led by Pinochet. Letelier, first jailed by the Pinochet regime and then stripped of his citizenship, particulaly criticized DINA and the Chileans for their alleged widespread human rights violations.

Goldberger underscored his claims about CIA involvement in the Letelier killing by giving the jury a brief history of recent U.S.-Chilean relations. He particularly noted the U.S. and-CIA expenditure of $13.4 million in Chile between 1963 and 1973 to block the election of and then to overthrow the Allende government.

Defense lawyers also started their effort to discredit Townley in other ways. Goldberger said flatly, "We will prove to you he's a hired killer. We will prove Michael Vernon Townley is a liar; his whole life has been a lie, one alias after another."

Goldberger also criticized Townley's plea bargain arrangement with the government. The government has promised to recommend that Townley be paroled after serving 40 months of a 3 1/3-to-10-year prison term.

Another defense lawyer, Lawrence Dubin, urged the jury to "keep asking why" Chile let Townley return to the U.S. "The questions are something (the government will) never be able to answer," Dubin told the jury as Propper and Assistant U.S. Attorney E. Lawrence Barcella Jr. objected.

As the prosecution and defense lawyers ended their opening remarks, the jury heard the first testimony in the case, a gripping account of the bombing by the lone survivor of the explosion, Michael Moffitt, an assistant to Letelier at the Institute for Policy Studies, a left-wing Washington think tank.

Moffitt and his wife, Ronni K. Moffitt, were riding to work with Letelier when the bomb, set off by a radio-control device, ripped through the floorboard of Letelier's Chevelle as he rounded Sheridan Circle near the Chilean Embassy. Letelier's legs were blown off and Ronni Moffitt was suffocated by her own blood as a result of injuries she received in the blast.

"Ronni and Orlando were talking about some book they had read when they were young," the husky 27-year-old Moffitt recalled. "As we got to Sheridan Circle, I was thinking about what a slow driver (Letelier) was."

Seconds later, Moffitt said, "I heard a hissing sound, a flash of light up toward the front of the car. The car just seemed to erupt in a deafening explosion. The noise was really deafening.

"There was huge hole in the car," Moffitt said. "Orlando was turned to the back of the car. His eyes were half open, half closed. I slapped him a couple times and said, 'Orlando, Orlando! It's Michael. Can you hear me?

"I managed to get my wrists under his arms," Moffitt said, and then realized that the bottom half of Letelier's body had been blown off

Moffitt then spotted his wife along the street curb and a woman passerby who was trying to help her. "The blood was just pouring out of her mouth," Moffitt said of his wife, whom he had married 3 1/2 months before.

Ronni Karpen Moffitt's parents, Murray and Hilda Karpen, and two brothers of hers, Michael and Harry, watched the proceedings in Judge Barrington D. Parker's courtroom yesterday. As Moffitt left the courtroom, Mrs. Karpen kissed him lightly on the cheek.

Defense lawyers will cross-examine Moffitt this morning. The defendants sat solemnly through Moffitt's testimony and showed no emotion. Two of the Cubans, Guillermo Novo and Ross, are charged with the murders of Letelier and Moffitt, while Ignacio Novo is accused of lying to a grand jury investigating the slaying and failing to tell federal officials about the crime.