American-born Chilean secret police agent Michael V. Townley calmly admitted to a federal judge here yesterday that he planted what he called the "device" that exploded to murder former Chilean ambassador Orlando Letelier and an aide in Washington in September, 1976.

Guarded by at least 12 deputy U.S. marshals and three FBI agents in a courtroom that included members of the Cuban nationlist group Townley said helped him carry out the assasination, Townley became the first person to plead guilty to the bombing murder of Letelier and Ronni K. Moffit.

The plea-bargaining agreement that brought Townley to court yesterday requires him to be sentenced to a term of 3 1/3 to 10 years in prison, with the government promising to recommend parole after he completes 40 months of the sentence. Although the plea bargaining agreement was acepted by the judge, the sentence is to be formally imposed later.

Townley, who is cooperating with U.S. law enforcement authorities is in the protective custody of U.S. marshals pending his final sentence.

U.S. Atorney Earl J. Silbert, who presented the plea arrangement to a reluctant U.S. District judge Barrington D. Parker, said he agreed to the sentencing plan because Townley was an "indispensable witness" against three Chilean secret police (DINA) officials and five American-based Cuban exiles also charged in the case.

"Without this witness, the government would not be able to support the indictment? Parker asked.

"That is correct," replied Silbert. "He's an indispensable witness." He said Townley's testimony was essential to solve the "heinous crime" and to show that U.S. authorities would take whatever steps they must to solve "terroristic attacks . . . whether committed by individuals, organizations or governments."

When first informed secretly by prosecutors and defense attorney Seymour Glanzer of the plea arrangement last week, Parker had delayed aceptance of it pending further review on his part. Yesterday, however, Parker finally said he would "reluctanly" accept the plea "in the interest of justice."

Federal judges here normally exercise sold discretion over sentencing, and are said to dislike sentencing agreements in general.

Parker's decision to accept the plea was announced after an hour-long hearing in which Townley, a lanky, bearded 35-year-old man in a dark blue suit clearly and coolly described for the first time publiciy his central role in building and placing the bomb that would violently blow Letelier apart. Townley, who said he had lived primarily in Chile since 1957, answered numerous questions from Parker about his assassination trip to the U.S. in explanatory, complete sentences rather than with short "yes" or "no" replies.

"I was an agent for DINA," Townley said at one point. "I was issued orders to complete a mission . . . the assassination of Orlando Letelier."

He said he flew to the United States from Chile, met with members of the Cuban Nationlist Movement (he used the Spanish name of the right-wing group) and began taking steps to murder Letelier. At least three members of CNM and wives and friends of other CNM members charged in the case watched Townley testify in the courtroom.

Townley spoke often the "device" and "elements" used in building the bomb in Washington. The term "explosives" came up only once, when Parker asked Townley if he had had previous experience with blasting materials, and Townley replied, "Yes, your honor."

Townley said he placed the device himself and largely assembled it by himself, but that he had left the Washington area by the time it was detonated.

He said he knows the other persons charged in the murder conspiracy with him, incuding former DINA chief Manuel Contreras Sepulveda and DINA operations chief Pedro Espinoza Bravo. The United States has said it will soon begin extradition proceedings against those tow, as well as DINA agent Armand Fernadez Lari-Letelier investigation.

During the course of the hearing yesterday, Townley's attorney, Glanzer, said that before Townley decided to enter his plea he met in the United States with Chilean officials who released him from orders of silence with directed him to cooperate in the Letelitr investigation.

The plea-bargainging agreement signed by Townley says that in return for his truthful testimony the United States will provide protection for his wife, children, parents and brother and sister while they are in the United States.

Townley, who has reportedly been involved in terrorist activities in other countries, only must provide information about acts involving U.S. citizens or occurring in the United States, according to the plea bargaining agreement.