SAN SALVADOR, NOV. 24 -- Salvadoran authorities today took a first step toward filing a case against rightist leader Roberto d'Aubuisson in connection with the 1980 assassination of archbishop Oscar Arnulfo Romero and ordered the arrest of an alleged accomplice living in Miami.

Judge Ricardo Zamora Perez, whose criminal court is handling the Romero case, formally asked the National Election Council to certify that d'Aubuisson is an elected deputy. Justice Minister Julio Alfredo Samayoa said the move was a step toward asking the legislature to strip d'Aubuisson of his immunity so he can be charged with having ordered the murder.

At the same time, Zamora ordered the arrest of Alvaro Saravia, a cashiered Salvadoran Air Force captain, on charges of aggravated homicide for allegedly organizing the assassination. The judge also ordered that Saravia's assets in El Salvador be frozen.

The arrest order is a prerequisite for a request to U.S. authorities to extradite Saravia, who was arrested in Miami last night. {The U.S. Marshal Service said it acted on a request by the Immigration and Naturalization Service, adding that information indicated his temporary residence permit had expired 10 months ago.}

The legal actions against d'Aubuisson and Saravia stem from new testimony made public yesterday by President Jose Napoleon Duarte. In the testimony, Amado Antonio Garay, 37, said he drove a getaway car for the assassin who shot the archbishop as he was saying mass in a hospital chapel on March 24, 1980. Garay testified that d'Aubuisson ordered the murder and that Saravia was involved in carrying it out.

In response to the accusations, d'Aubuisson charged today that the Romero killing and other death-squad activities in the early 1980s were the work of National Police under the command of Col. Reynaldo Lopez Nuila.

Lopez Nuila, an ally of Duarte who formerly headed the National Police, is currently vice minister for public security. He has come under sharp criticism from military colleagues for trying to establish a professional police force independent of the Army.

Today's legal moves came a few hours after U.S. Ambassador Edwin Corr met with the prosecutor in charge of investigating the Romero case, Attorney General Roberto Giron Flores, and with the president of the Supreme Court to discuss the next steps in the case.

Meanwhile, d'Aubuisson's right-wing political party, the Republican Nationalist Alliance (Arena), vehemently denounced the accusations against its founder and honorary president-for-life, calling them "calumnies and insults" that are likely to "foment violence." An Arena deputy, Armando Calderon Sol, accused Duarte of "politicizing the Romero case" to camouflage the "moral decomposition of his party" and the "corruption and ineptitude" of his government.

Calderon Sol also charged that former U.S. ambassador Robert White, who implicated d'Aubuisson in Romero's murder during congressional testimony three years ago, was "a proconsul sent by the socialists in Washington to promote social change in El Salvador and destroy our economy." He added that d'Aubuisson has "proof of his innocence" and will present it in court if the case comes to trial.

However, knowledgeable sources said that even if the National Assembly, which is dominated by Duarte's Christian Democratic Party, strips d'Aubuisson of his immunity by a majority vote, he is unlikely to be convicted by a Salvadoran court. In the event of a trial, the court of first instance in d'Aubuisson's case would be an appeals court, and the majority of judges at that level are supporters of Arena, the sources said.

The timing of the disclosure of Garay's testimony struck many observers here as politically motivated in view of the return to the country Saturday and Monday of two exiled leftist leaders associated with El Salvador's rebels. But the content of the testimony appeared to tally with some previous disclosures about the Romero case.

Garay said an unidentified bearded assassin killed Romero with a single shot from a rifle, which he fired through the open door of the Divine Providence Hospital chapel from the back seat of a red Volkswagen Garay was driving.

An initial report of Romero's murder in 1980 had said four men had walked up the aisle of the chapel and killed him. But this version was quickly discounted when it became clear that none of the worshipers had actually seen the gunman.

A Carmelite nun who was working in an office next to the chapel on the day Romero was killed said today that "nobody saw anybody enter the church" to kill Romero. She said Romero was finishing a homily in front of the altar when a single shot rang out and the archbishop fell dead from a bullet wound near the heart. "Nobody saw the assassin enter," said the nun, who did not want to be identified out of fear for her safety.

From a road inside the hospital compound that passes in front of the chapel, there is an unobstructed view of the altar when the doors are open. About 30 yards separate the road from the altar, which sits on a raised marble area at the end of a wide aisle.

According to documents presented in U.S. congressional hearings in 1981, Saravia was part of a group recruited in 1979 and 1980 by d'Aubuisson, then a major in charge of National Guard intelligence, to help fight a Marxist-led insurgency through use of kidnapings and assassinations.

According to one document released in the Senate Foreign Relations Committee hearings, a diary that belonged to Saravia, the group in early 1980 bought more than 70 weapons, including sniper rifles and eight telescopic scopes. Also among the group's property was a "red car," according to the diary.

Garay said Romero was shot at dusk from a red Volkswagen he was driving.

A separate document that sought to explain the notations in the diary concluded that "Archbishop Romero was killed by a Salvadoran sharpshooter hired by d'Aubuisson."

The diary was among documents seized May 7, 1980, when d'Aubuisson and a group of friends were arrested on charges of plotting a coup.

D'Aubuisson was cashiered from the National Guard but was later released from detention by a military judge who found a lack of evidence against him in the alleged coup plot.