SAN SALVADOR, OCT. 27 -- The murder of the head of a leftist human rights group here yesterday has aroused fears of renewed activity by right-wing death squads and raised a potential obstacle to Central American peace efforts.

Herbert Anaya, 33, the president of the Human Rights Commission of El Salvador, was shot to death outside his home yesterday morning. Police said the killers, two unidentified men carrying 9-mm pistols, fled the scene.

Although no group has claimed responsibility for the killing, leftist opposition and human rights groups attributed it to the kind of death squads that murdered hundreds of people a week in the early 1980s but that seem to have been largely dormant in recent years.

To protest the killing, about 200 members of leftist groups including the human rights commission and the Committee of Mothers of the Disappeared of El Salvador, known as Comadres, marched at dusk with Anaya's coffin from the cathedral to the U.S. Embassy to hold a vigil. Behind a pickup truck bearing the coffin, one marcher carried an effigy of Uncle Sam that was later burned. Anaya's group has accused the United States, which provides substantial aid to the Salvadoran government and military, of complicity in the murder.

In a separate protest statement, the National Union of Salvadoran Workers said it would observe a "day of national indignation" Wednesday. The Marxist-led Farabundo Marti National Liberation Front guerrilla organization announced that it would soon call a national transportation strike.

The government has accused the human rights commission, Comadres and the National Union of Salvadoran Workers of being fronts for the guerrillas, a charge the groups have denied.

The murder of Anaya coincided with efforts by the government of President Jose Napoleon Duarte to push through the National Assembly a controversial amnesty law for political crimes and to demonstrate compliance with a Central American peace accord.

Diplomatic sources and independent human rights activists said the effect of the killing could be to set back the peace process in El Salvador, scaring off rebel fighters and political leaders who might want to accept amnesty.

The amnesty proposal being debated in the National Assembly broadens the definition of political crimes from offenses against the state to all crimes associated with the country's eight-year-old armed conflict except kidnaping, extortion and drug trafficking. Critics said this would absolve both the perpetrators of such crimes as guerrilla murders of U.S. servicemen, and massacres of civilians by death squads and military men.

According to figures compiled by Tutela Legal, the rights office of the Roman Catholic Archdiocese of San Salvador, murders attributed to death squads have declined dramatically since tens of thousands of people were killed in the early 1980s. Of more than 60,000 people estimated to have been killed in El Salvador's civil war, at least 40,000 were victims of death squads, human rights groups say.

Indicative of the decline in these killings, however, Tutela Legal reported 136 death squad murders in 1985 and 45 in 1986.

Human rights activists have charged that the Salvadoran death squads are made up of members of various branches of the armed forces and security services operating in civilian clothes. Although the death squads were reported to have been largely disbanded under U.S. pressure, it is widely believed that the ability exists to reconstitute them quickly.

"We've never seen any investigation or punishment of anyone who was associated with a death squad," an independent human rights official said. "The deaths squads still have the ability to get somebody if they want to."