WASHINGTON (AP) _ A former Chilean secret police official pleaded guilty today to aiding in the 1976 car bombing on Embassy Row that killed former Chilean ambassador Orlando Letelier.

Armando Fernandez Larios, who resigned Monday as a major in the Chilean army and as a member of its intelligence agency, pleaded guilty to one count of being an accessary after the fact to the murder of Letelier, who died when a bomb in his car exploded on Sept. 21, 1976.

The explosion also killed Ronni Moffitt, an associate of Letelier's at the Institute for Policy Studies, a liberal Washington think tank.

Fernandez could receive a sentence of up to 10 years in prison. U.S. District Judge Barrington Parker rejected a provision of the plea agreement between Fernandez and federal prosecutors that would have limited his sentence to seven years.

The judge set April 6 as the sentencing date for Fernandez.

Fernandez, who has agreed to tell federal prosecutors all he knows about the car bomb slaying, was one of four Chilean secret police officials indicted in 1978 on charges relating to the car bombing slaying.

But the Chilean government in 1979 refused U.S. extradition requests for Fernandez and two of his co-defendants.

Fernandez admitted during a two-hour grilling by Parker that he lied at his extradition hearing and before a Chilean magistrate to impede the investigation of Letelier's assassination. Fernandez said he told a ''cover story'' at these judicial proceedings and to U.S. investigators because he was ordered to do so by superiors in the Chilean military.

He testified that Gen. Augusto Pinochet, the head of the Chilean government, visited him in 1979 and told him that his problem would be solved. ''He said 'Fernando, I know that you have some problems,''' Fernandez said. ''He said, 'You are a good soldier. You must not worry about this. This is going to be all right.'''

Asked by Parker to describe Pinochet's demeanor, Fernandez said, ''I think he was a little worried.''

But Fernandez said he never specifically discussed his trip to Washington with Pinochet.

U.S. Attorney Joseph diGenova said after the hearing that the Justice Department had requested the extradition of two other Chilean military officers named in the 1978 indictment: Retired Gen. Juan Manuel Contreras Sepulveda and Lt. Col. Pedro Espinoza Bravo.

Isabel Morel Letelier, the former diplomat's widow, said Fernandez's guilty plea ''gives satisfaction to hear a person repeat what we have been saying for 10 years.''

''It gives satisfaction to see a young officer of the Chilean army not be able to live with the burden of crimes and schemes and lies and false passports.''

But Mrs. Letelier said the people who ordered her husband's assassination are still in power.

Charles E. Redman, the State Department spokesman, said earlier today that Fernandez came to the United States voluntarily. ''He advised us he suffered remorse for the deaths of Orlando Letelier and Ronni Moffitt, and that he considered the murder and the subsequent cover-up dishonored Chile and its military institutions.''

The U.S. official said Fernandez proposed coming to the United States to make a full disclosure of his own role and that of others.

Redman said Fernandez was offered protection under a federal law designed to protect witnesses.

Another Chilean secret police official, American-born Michael V. Townley, pleaded guilty in 1978, admitting that he planted the bomb on Letelier's car. Townley implicated Fernandez and the other Chileans.

Fernandez told Parker during the hearing that he traveled to Washington in the summer of 1976 and found the home and office of Letelier. Fernandez said he determined that Letelier was out of the country, and Fernandez later returned to Chile.

But Fernandez said he met Townley at New York's Kennedy International Airport and gave him the addresses of Letelier's home and office and gave him a tourist map of Washington.

Townley, who served a prison term of three years and four months, had told federal officials that Fernandez conducted a surveillance of Letelier's movements in Washington before the bombing.

Letelier had served as Chile's ambassador to Washington and foreign minister under the Marxist government of the late President Salvador Allende. After a military coup toppled Allende in 1973, Letelier was jailed by the new government.

He later went into exile in the United States, where he became an active critic of the Chilean military regime.