A "substantial number" of witnesses for the prosecution against eight men charged in the bombing death of former Chilean ambassador Orlando Letelier have told the government they fear "physical retaliation," according to court documents filed here.

Some witnesses already have been placed under protection of United States marshals, although federal prosecutors declined yesterday to say exactly how many. Two of the witnesses known to be under protection of marshals are Michael. V. Townley, who has admitted planting the bomb that killed Letelier and an aide in September 1976 and Townnley's wife, Mariana Callejas de Townley.

The documents, filed in response to motions made by defense lawyers, provide some new details on a case where prosecutors, acting under court order not to discuss the case, have been relatively silent.

In the course of arguing against defense requests for evidence gathered by the government, prosecutors asserted that "there is no evidence whatsoever that the Central Intelligence Agency had either knowledge of or participated in the Letelier assassination." While it may be a popular pastime and interesting cocktail party conversation to level unfounded charges at the Central Intelligence Agency," one document states, "there is not the slightest scintilla of evidence to indicate CIA involvement or knowledge of this matter."

According to the prosecution memorandum, Townley had two brief, "unrelated" contacts with the CIA in 1970 or 1971 and again in 1973 - both initiated by Townley. "These two incidents represent the sum total of contacts between the CIA and Mr. Townley," the memo says.

The government memos offer additional information about the nature of the relationship between Townley and the other defendants. Townley did not, one of the government memos states, simply contact four of the defendants "'out of the blue'" on Sept. 10, 1976 - 11 days before Letelier and an aide, Ronni Moffitt, were blown up in an auto on Sheridan Circle. Townley had a "pre-existing relationship" with five of the defendants, according to the memO.

Of the eight defendants charged in connection with the bombing, three are in custody, two are being sought by the FBI and the remaining three are Chileans whose extradition is being sought by the United States.

The United States formally requested extradition of Gen. Juan Manuel Contreras Sepulveda, former head of Chile's old secret police force, known as DINA; Pedro Espinoza Bravo, who was DINA's director of operations when Letelier was killed, and Armando Fernandez Larios, a DINA agent who allegedly helped arrange the assassination. All three are under military arrest while the president of the Chilean Supreme Court decides whether to extradite the three.

The trial is scheduled to begin in January. Assistant United States Attorney Eugene M. Propper said yesterday that the trial would proceed on schedule, whether or not the three men are extradited and whether or not they are extradited "on time."

Propper declined to say whether any of the prosecution witnesses have actually been threatened or whether they simply have expressed general concern and anxiety about their safety.

The government also is opposing an effort by the defense to change the locale of the trial from Washington on the grounds of excessive pretrial publicity. Comparing the publicity in the Watergate trial to this case, the government argued, is, "In sheer terms of weight, like comparing a whale to a minnow" and yet no change of location was granted for the Watergate trials.