SAN SALVADOR, JAN. 3 -- Archbishop Arturo Rivera y Damas, leader of El Salvador's Roman Catholic Church, said today in a televised mass that death squads were returning and that this "nefarious sin" should be eradicated.

"In recent days I have had the opportunity to meet with almost all of the bishops of the country," the prelate said. "They expressed to me their concern over the tendency toward the return of the death squads."

"I ask that {in} this year of grace 1988 we end this practice, which threatens the consolidation of a true democracy as well as being a nefarious sin that goes against the Christian spirit," he said.

"It is a phenomenon that is real and we will continue to denounce it and bring what pressure we can," the archbishop said in a brief interview after the mass. He has been criticized in the past for not speaking out on human rights abuses.

Last week, Auxiliary Bishop Gregorio Rosa Chavez said a major challenge was "how to stop the resurgence of the death squads, which appear here cloaked in darkness."

Right-wing death squads killed tens of thousands of suspected leftists in the early years of the civil war between the U.S.-backed government and a Marxist-led insurgency. Evidence has linked those squads to members of the security forces.

Last month, a regional leader of the governmental human rights commission was assassinated 80 miles east of here, and in October the president of the nongovernmental commission was shot dead in the capital.

Rivera y Damas' predecessor, Oscar Arnulfo Romero, an outspoken critic of the killings, was gunned down in 1980 as he said mass. The government cited new evidence last month to charge that rightist leader Roberto d'Aubuisson was responsible for the killing. He denied the charge.

Three weeks ago, Rivera y Damas said he had received telephoned death threats from anonymous callers. About 25 civilians have been killed during the past three weeks, but only a handful of those murders have had obvious political overtones.

Most have been carried out in typical death-squad fashion, however -- the victims were gunned down with automatic weapons from a passing car, shot at close range or their throats were slit after they were picked up and tortured.

Asked if he had spoken to the military about the church's concern, Rivera y Damas said, "Not yet."

The killings dropped off sharply following the election of President Jose Napoleon Duarte in 1984, when some of the officers most closely linked to killings were removed from the military or sent out of the country on diplomatic missions. No officers have ever been tried for human rights abuses.

The United States provides $1.5 million a day in economic and military aid to El Salvador, giving it leverage with the Army, security forces and government. But critics of U.S. policy have long charged that while killings have dropped dramatically, the structure of the death squads remained in place. Several observers who know the military well said they believed the death squads operating now have no official sanction. But many fear a tolerance of "freelance" killings.

"There has been a loosening of the reins, especially among the mid-level officers," one civilian who works closely with both the government and military said in an interview last month. "No one is giving orders that people be killed, but they are more willing to turn a blind eye now than a month ago, because they feel more threatened."

The most recent documented charge of military torture and killing occurred in early December, when the 1st Infantry Brigade arrested Gerardo Hernandez Torres, a baker, on suspicion of being a guerrilla and turned him over to the National Police.

"He died in Mariona prison, and the autopsy clearly shows he died from the torture he was subjected to," Rosa Chavez said at the time. "Does this mean we are returning to the dark methods of years gone by? We must remember we cannot think about peace without respect for human rights."