In a calm, matter-of-fact fashion, American-born Chilean secret police agent Michael Vernon Townley told a federal court jury here yesterday of his career as an international terrorist and how he built and planted the bomb that killed former Chilean ambassador Orlando Letelier.

As a packed and rapt courtoom audience listened for 2 1/2 hours, the 36-year-old Townley recounted detail after detail of how Letelier's car was blown up as he drove along Washington's Embassy Row on the moorning of Sept. 21, 1976 -- the most notorious act of international terrorism ever carried out in the nation's capital.

As Townley waited to testify under heavy guard from U.S. marshals, he was greeted with a chorus of invective in Spanish from the three staunchly anti-Castro Cuban exiles on trial in connection with the slaying of Letelier and a colleague of his at the Institute for Policy Studies, Ronni K. Moffitt.

The defendants -- Guillermo Novo Sampol; his brother, %ignacio Novo Sampol, and Alvin Ross Diaz -- variously called Townley a "traitor" and a "degererate." Someone in the court room said, "His tongue should be cut out."

The defendants' comments were made while their lawyers, U.S. District Court Judge Barrington D. Parker and the jury of seven women and five men were out of the courtroom. Marshals seated directly behind the defendants apparently did not understand the Spanish phrases. Townley, seated about 20 feet away, glanced once at the defendants, but did not respond to their taunts.

As he took the witness stand, Townley coolly recounted how he ran a clandestine radio station in Chile during the regime of Marxist president Salvador Allende in the early 1970s, led an abortive assassination mission to Mexico in 1975 as a secret police agent, sent explosives via unsuspecting LAN Chile airline pilots to Cuban exiles in the United States and finally helped plt the killing of Letelier.

Townley stated his marching orders from Chilean secret police operations director Pedro Espinoza Bravo in simple terms: "The elimination or assassination or killing of Orlando Letelier... to kill him, making it look like an accident, a suicide, as innocuous as possible."

With that mission in mind, Townley recalled the use of multiple aliases to enter the United States, the recruitment of Cuban exiles to help carry out the assassination, the construction of the bomb in a hotel room along New York Avenue NW, the difficulty in placing what he called "the device" underneath Letelier's car outside the former diplomat's Bethesda home and then the wait for the bomb to be detonated by radio control.

The tall, broad-shouldered Townley, dressed in a blue, three-piece business suit with muted blue and red stripes, testified in such a flat tone of voice that he could have just as easily been describing a Sunday drive in a park.

He pleaded guilty to the murders of Letelier and Moffitt last August and, under his plea bargaining agreement with federal prosecutors, will eventually be sentenced to a 3 1/2-to-10-year prison term, with the government promising to recommend parole after he serves 40 months of the sentence.

The three Cubans on trial are among eight defendants charged in connection with the killings, but two Cuban exiles -- Virgilio Paz Romero and Jose Dionisio Suarez Esquivel -- are fugitives. Three former DINA officials are awaiting a decision from the Chilean Supreme Court on whether they are to be extradited to the U.S. to stand trial for the killings.

Many people in Chile and the U.S. familiar with the case view the ultimate defendant as the government of Chilean military dictator Augusto Pinochet, who led a junta that overthrew Allende in a bloody coup in September 1973. Letelier and other Allende officials and supporters were imprisoned and no charges were placed against them. Letelier, a strident critic of Pinochet and the alleged human rights violations his government has committed, was eventually expelled fromh expelled from the country and stripped of his citizenship the same month he was killed.

Townley testified that Juan Manuel Contreras Sepulveda, former director of the Chilean secret police, once known as DINA, who is one of the three Chileans indicated in the case, told him a year ago that "no one else above him knew about" the plot to assassinate Letelier.

Nonetheless, friends and former colleagues of Letelier's have long believed that Pinochet himself ordered the assassination, in part because of their belief that Contreras would not have acted on his own, without the permission of his close associate, Pinochet.

Townley also testified that he never worked for the Central Intelligence Agency, a contention that the defense lawyers, when they get their chance to cross-examine him today, are certain to contest in every way possible. The government has contended that DINA ordered Letelier's assassination.But the defense is trying to prove that Townley was a double agent, working both for DINA and the CIA and that it was the CIA that directed Letelier's slaying.

Townley said he called the CIA to offer his services in 1970 when he was returning to Chile after a four-year stay in the U.S., was visited by a CIA agent that year and then twice called the CIA offering information in 1973. However, a CIA affidavit submitted in the case states that an arm of the agency once sought to use Townley in an operational capacity.

Townley said the he began his anti-Allende efforts in October 1972 and was particularly active in the Patria y Libertad, an extreme right-wing group that spearheaded terrorist actions against the Allende government. It was in that period that Townley said he established the clandestine radio station that broadcast anti-Allende propaganda.

Townley said he left Chile in late March 1973 because "I was being sought by the Allende government for my activities." But he said he returned in October 1973, a month after the Pinochet coup.

He said he met Espinoza, the Chilean secret police director, in April or May 1974 and developed a "superficial friendship," during which Espinoza suggested that Townley, with his life-long interest in electronics, "might be of service to the intelligence service."

Townley said he started working part-time for DINA in October 1974 and joined the secret police on a full-time basis two months later.

"I was given a mission to go to the U.S.," Townley said, adding that he entered his own country using the alias Kenneth William Enyart, one of four aliases he used at various times as DINA agent.

His mission, he said, was to purchase various kinds of electronic equipment. Later, he said he was given the assignment to travel to Mexico City to "eliminate" two Chilean exile leaders, Carlos Altamirano and Volodia Teitelboim, and "disrupt an upcoming meeting of Chilean exiles."

Townley said he went to Miami to recruit Cuban exiles to assist in the Mexico effort, but that eventually he, his wife Mariana, another DINA agent and Pax arrived "one day too late to carry out the primary mission."

For their help in the abortive Mexico assassination trip, Townley said he explosives to members of the Cuban Nationalist Movement in New Jersey via "friendly" LAN Chile pilots who did not know the contents of the packages.

In later June or early July 1976, Townley said, Espinoza asked him to undertake another mission, the killing of Letelier. Townley said that at first he and another DINA agent indicted in the case, Armando Fernandez Larios, were sent to Paraguay to try to secure false passports for the mission to the U.S. Although they got the passports, Townley said they went back to Chile because they feared they bad been discovered by the CIA.

Later, Townley said he was told that Fernandez had been sent to the U.S. to tail Letelier and find out where he lived and worked. Townley then was ordered to the U.S. to recruit Cuban exiles to help with the assassination.

Townley said he met in New Jersey with Guillermo Novo, Suarez, Paz, Ross and others to persuade them to take part in the killing.

While reluctant at first, Townley said the Cubans eventually agreed, as long as Townley took part himself. Townley said that Paz, Suarez and Guillermo Novo supplied some of the bomb-making materials and that he had brought other parts from Chile and passed through U.S. customs with them.

Townley said that he and Pax traveled to Washington and later were joined by Suarez. After building the bomb at the Regency Congress Hotel, Townley said that he planted it underneath Letelier's car early on Sept. 19, 1976.

"I had a very hard time fixing the device," he recalled and said that later he learned that Pax and Suarez had to return to adjust the bomb to make sure it would detonate.

By the time the bomb was exploded by a radio control device, Townley was in Miami and quickly returned, just as he had been ordered, to Santiago, Chile.