While barking German shepherd dogs sniffed for bombs outside a federal courtroom, the trial started yesterday of three fervent anti-Castro Cuban exiles charged in connection with the 1976 bombing assassination here of former Chilean ambassador Orlando Letelier.

U.S. District Court officials imposed their strictest security ever, forcing everyone entering the courtroom to walk through two metal detectors and scanning their personal belongings with an X-ray machine.

In part, the security was a reflection of the passions that have been and still are stirred by the Embassy Row slaying of Letelier, one of the most ardent critics of the current Chilean military dictatorship of Gen. Augusto Pinochet. His killing, the most notorious act of international terrorism ever committed here, shocked Washington in September 1976. The indictment in the case alleges that the Chilean secret police, once known as DINA, masterminded the plot to kill Letelier.

Yesterday, former colleagues of Letelier and Spanish-speaking relatives and friends of the three defendants nearly filled Judge Barrington D. Parker's courtroom. They listened as defense and prosecution attorneys sparred over preliminary legal issues that gave but a hint of the drama of international intrigue that is expected to unfold over the next five to six weeks.

At one point in yesterday's proceeding, defense lawyer Paul Goldberger mentioned that the prosecution had informed him that the government's key witness, Michael V. Townley, the American-born, Chilean secret police agent who has confessed to planting the bomb in Letelier's car, was also possibly involved in a kidnaping plot with "one or more" of the Cuban defendants on trial.

But Parker quickly cut off discussion of the matter and it was not clear when or where the plot might have taken place or who was the intended kidnap victim.

At another point, Goldberger mentioned a mysterious telephone call he received last week at his New York law office when a caller cryptically told him, "Don't press the CIA thing" and then hung up. Defense lawyers for weeks have been attempting to link the Central Intelligence Agency to the slaying of Letelier, who served in the Marxist government of Salvador Allende until he was overthrown by the Pinochet Junta.

By day's end, Judge Parker had ordered the government to turn over the FBI's original notes that agents took during more than two weeks of debriefings of Townley last April, four months before he pleased quilty to the assassination. In addition, the judge dismissed 28 prospective jurors who told him privately at the bench that for medical or personal reasons they could not serve on a jury that will be sequestered in a local hotel for the lengthy trial.

Parker ordered an additional 125 prospective jurors to return this morning for more questioning in what he called "the time-consuming and painstaking" task of picking 12 jurors and an unspecified number of alternates to hear the case.

The three defendants -- Guillermo Novo Sampol, his brother Ignacio Novo Sampol, and Alvin Ross Diaz -- frequently talked among themselves and their lawyers during the proceedings and waved and smiled at their supporters in the courtroom.

A dozen of the defendants' supporters, mostly Cuban exiles from Union City, N.J., stood silently when Ross and Guillermo Novo were led by marshals into the courtroom. One of the supporters said that about 200 Cuban merchants in Union City closed their stores yesterday in a show of support for the defendants.

Ross' daughter, Anna Muller, and Letelier's daughter, Fabiola, sat in the courtroom, as did Saul Landau, Letelier's successor as director of the Transnational Institute of Policy Studies, a foreign affairs affiliate of the Institute for Policy Studies.

Guillermo Novo and Ross are charged with the murder of Letelier and Ronni K. Moffitt, a collegue of Letelier at the Institute for Policy Studies who was riding to work with him when Letelier's car exploded on the morning of Sept. 21, 1976. Ignacio Novo is charged with perjury in connection with testimony he gave to a grand jury investigating the case and failing to tell federal officials about the crime.

Despite the byplay among lawyers at a two-hour morning session and the three hours of questioning of prospective jurors in the afternoon, the courtroom events were overshadowed by the unprecedented security imposed at the courthouse at Constitution Avenue and John Marshall Place NW, a short distance from the Capitol.

Visitors to the courthouse were required to pass through one magnetometer when they entered the building and then a second one before they entered Parker's spacious, sixth-floor courtroom. In addition to sending their belongings through the X-ray machine, visitors were required to sign a register and give court officials some personal identification while they witnessed the courtroom proceedings.

At least three sweeps of the courthouse by the teams of dogs turned up nothing, according to U.S. Marshal J. Jerome Bullock. But court officials clearly were nervous about the prospect of a terrorist act.

At one point yesterday afternoon, an FBI agent using the sixth-floor men's room discovered a piece of paper with a small circular emblem saying "Omega 7" on it.

Court officials immediately sent the dogs on another sweep of the building because an anti-Castro group calling itself Omega 7 took responsibility for two bombings Dec. 29 in New York City' One of those bombs was detonated at Lincoln Center a short time after the Cuban national ballet had performed there and the other exploded at the Cuban mission to the United Nations. Court officials said the search turned up nothing.

Parker quickly gave notice that courtroom decorum during the trial would be as strict as the courthouse security. Just moments after he convened the proceedings, he admonished Goldgerger and one of the government's prosectors, Assistant U.S. Attorney Eugene M. Propper, for allegedly violating his order against talking about the case with reporters.

Later, when Propper told Parker that it was not the usual practice in District of Columbia courts to turn over investigators' original notes of interviews with government witnesses to defense attorneys, Parker snapped at him, "Now Mr. Propper, don't play games. You're playing games."

Parker then ordered the government to turn over to defense attorneys the FBI agents' notes of interviews with Townley. The government has already given defense attorneys a summary of Townley's statements to investigators as well as his grand jury testimony.

Parker said he would reconsider his previous denial of a defense request to obtain the contents of Letelier's breifcase found in the wreckage of his Chevelle. But Parker denied for the second time a bid for a change of venue.

The judge scheduled a hearing for today on whether evidence seized by FBI agents at Ross' former office in Union City, N.J., should be admissible at the trial.