The confessed assassin of former Chilean ambassador Orlando Letelier threw the trial of three Cubans charged in connection with that murder into a new legal dispute yesterday by refusing to answer a question about his activities with the Chilean secret police.

Michael Vernon Townley, an American-born agent for the Chilean secret police once known as DINA, is the government's key witness against the three staunchly anti-Castro Cuban exiles on trial.

He told U.S. District Court Judge Barrington D. Parker that he was invoking the Fifth Amendment right against self-incrimination and refusing to answer a defense lawyer's question about where specifically he obtained so-called electric matches, one of which was used to help detonate the bomb that killed Letelier along Washington's Embassy Row on Sept. 21, 1976.

"Due to the nature of my work," the 36-year-old Townley said, without elaborating, "I can't tell you because of legal responsibilities I might incur in Chile. I must respectfully decline to answer the question on grounds that it might incriminate me."

Moments before, Townley told defense lawyer Paul Goldberger that he obtained the matches "from my service," meaning DINA, the Chilean secret police on whose orders he has testified he killed Letelier.

But when Goldberger asked Townley where in DINA he had gotten the matches, the international terrorist refused to answer

After a brief bench conference, Parker said that "with a great deal of regret" he was forced to send the jury of seven women and five men back to their hotel for the weekend. The judge ordered defense and prosecution lawyers to submit legal briefs to him today stating their positions on whether Townley can selectively invoke the Fifth Amendment and whether he can do so to avoid self-incrimination in a foreign country.

Townley has already pleaded guilty to murder a foreign official and eventually will be sentenced to a 3 1/2- to 10-year prison term. In exchange for his testimony, the government has agreed to recommend that he be paroled after serving 40 months of the sentence and not prosecute him for other offenses.

As a result of Townley's refusal to answer the question, defense lawyers asked Parker to strike his prosecution testimony from the record. During 2 1/2 hours on the witness stand Thursday, Townley matter-of-factly and in great detail described his career as a rightwing terrorist and his role in plotting Letelier's assassination. Parker did not immediately rule on the defense motion.

Because of a variety of legal disputes yesterday and on Wednesday and Thursday, the jury has heard only four hours and 20 minutes of testimony in the last three days. The rest of time the jurors have been kept out of the courtroom or sequestered in their hotel.

In his testimony yesterday, Townley described the mechanical aspects of how the bomb was detonated and also confirmed the legitimacy of a variety of prosecution documents tracing his travels from Chile to New York to New Jersey to Washington, just before the bombing of Letelier. The blast also killed Ronni K. Moffitt, an aide of Letelier's at the Institute for Policy Studies.

In other developments yesterday:

Parker told defense lawyers that he would not permit them to question Townley about terrorist acts he allegedly committed against Chilean exiles in Argentina and Italy. According to sources familiar with the investigation, Townley participated in the 1974 Buenos Aires bombing that killed retired Chilean Gen. Carlos Prats and his wife in an assassination similar to Letelier's and the 1975 machine gunning in Rome of Chilean exile Bernardo Leighton and his wife.

Defense lawyers argued strenuously that they needed to ask Townley about these attacks in order to give the jury the full picture of Townley's activities. But the government's plea bargain agreement with Townley only requires him to discuss crimes in the U.S. or involving U.S. citizens.

Parker said the jury already had enough information about Townley's role in the Letelier killing and an abortive assassination trip to Mexico in 1975.

"Mr. Townley is certainly not a person you would like to sit next to at a Sunday worship service," the judge observed.

Defense lawyer Goldberger said that he will produce a witness who will testify that Townley had contacts with the Central Intelligence Agency in 1975 and 1976, not just in 1970 and 1973 as Townley testified. The defense has claimed that the CIA ordered the killing of Letelier.

Parker rebuked the three defendants -- Guillermo Novo Sampol, his brother, Ignacio Novo Sampol, and Alvin Ross Diaz for the invective in Spanish they taunted Townley with when he first appeared in the courtroom Thursday.

A Spanish-speaking U.S. marshal was assigned to sit behind the defendants yesterday to listen in case the remarks about Townley were repeated.