Thousands of people marched through the capital Monday on the sixth anniversary of the assassination of Archbishop Oscar A. Romero, demanding that the government bring his killers to trial.

Peasants, workers and students thronged San Salvador streets, carrying placards and portraits of Romero, an outspoken critic of right-wing death squads and government repression, who was gunned down in 1980.

"Justice and clearing up of the crime," said signs carried by marchers in the demonstration, which police said attracted 25,000 people.

Romero was killed by suspected rightist gunmen while celebrating mass at a hospital chapel at the beginning of the civil war that has taken the lives of 60,000 people in battle or in violence.

"We must insist that the government pursue this investigation, hear testimony and continue gathering proof," Archbishop Arturo Rivera y Damas, Romero's successor, told reporters last week.

Although President Jose Napoleon Duarte has reported recent advances, human rights observers say the slow progress that has characterized the investigation leaves little hope that Romero's slayers will ever be captured and prosecuted.

Duarte told a news conference last month that after years of searching, the government had tracked down and obtained key testimony by an American photographer who had apparently witnessed the assassination. But human rights activists contended that Duarte was pressing Romero's case to sustain U.S. economic and military aid, which is tied to the country's human rights record.

Several U.S. press reports have cited classified congressional documents linking one of El Salvador's most prominent right-wing leaders, former army officer Roberto d'Aubisson, to the archbishop's slaying.

Two Salvadoran army officers linked by U.S. officials to death squad activities were promoted in January despite State Department objections. "We have let the Salvadorans know our opposition to their promotion, but we know that if we push it too hard, we could create divisions within the army," one official said. "Frankly speaking, we could press the Salvadorans to bring these guys to justice, but it's just not a high enough priority," he added.

A Salvadoran human rights observer contrasted the slow process of the Romero inquiry with the relatively rapid conclusion of the June 19 slayings of four U.S. Marines.

"When it comes to the murder of Salvadorans, the Americans want investigations so long as the truth never comes out," one observer charged. "They want to defend human rights without punishing the people who violate them."