A call for peace talks from El Salvador's leftist rebels has provoked an angry backlash from this country's powerful right wing, dealing a setback to what seemed like improving chances for a negotiated end to the three-year-old civil war.

The political reversal has further soured the atmosphere here and compromised President Alvaro Magana's U.S.-backed efforts to channel leftist opposition from guerrilla warfare into a constitutional political process leading to elections scheduled for the spring of 1984.

As a result, prospects seem as clouded as ever for a swift end to the fighting, which in a three-week rebel offensive in eastern El Salvador has killed or wounded several hundred more guerrillas, soldiers and civilians this month.

"The word 'negotiations' has long been impossible here," U.S. Ambassador Deane R. Hinton said in an interview. "Now the question is whether the word 'dialogue' also will become impossible after all this."

"All this" began about two weeks ago when Magana received private peace proposals from the insurgent leadership, reportedly through officials of El Salvador's politically active Roman Catholic Church.

The feeler arrived in the name of the Revolutionary Democratic Front, the overall rebel political group, and the Farabundo Marti National Liberation Front, the umbrella organization of five antigovernment guerrilla groups.

Several recent developments formed a backdrop for the rebel proposals. First, the insurgents had launched on Oct. 10 their broadest offensive since elections last March, reminding the world that despite a loss of momentum they were still a force to be reckoned with. Second, Costa Rican officials had revealed in mid-September that they were trying to act as go-betweens to arrange peace contacts and that both Magana's government and the rebels seemed interested.

Perhaps just as important, Hinton had told an interviewer for the London Times that he was "relatively optimistic" about chances for such a dialogue, probably through third parties such as the Costa Ricans or the church.

The ambassador, a veteran diplomat, chose his words carefully. The Reagan administration and the Magana government make a distinction between "dialogue," which they interpret as contacts aimed at allowing the left to lay down its arms and get involved in the elections, and "negotiations," in which the guerrillas would bargain for a share of power.

Nonetheless, his remarks caused a storm here, generating attacks by leaders of the Chamber of Commerce and the Salvadoran Industrialists' Association. Both groups include businessmen and personages closely identified with El Salvador's extreme right, which considers the guerrillas "criminal terrorists" best dealt with by high-powered rifles.

"They were getting nervous, I guess, and they were ready to hype all this," an informed official said.

According to a high-ranking cleric with ties to the rebels, the reaction was particularly violent because of several earlier signs that a "nervous" right was reading as a softening of the U.S. stand here.

These were the departure of Alexander M. Haig Jr. as secretary of state and his replacement by the less vociferous George P. Shultz, coupled with speeches in San Francisco and Chicago by Assistant Secretary of State Thomas O. Enders that were seen here as more conciliatory than past U.S. positions on Central America.

The U.S. attitude here is regarded as the key to chances of success for almost any political group, including the government and the Army. Washington is providing $230 million in aid this year, including about 50 U.S. military advisers along with arms, equipment and special training considered vital to the Army's fight against the guerrillas. Both the right and the left, therefore, were monitoring U.S. statements carefully.

The exiled rebel leadership, while seeking to maintain its military offensive, began leaking reports of a big peace initiative. At the same time, rightist elements in the Salvadoran security forces, acting while Defense Minister Jose Guillermo Garcia was out of town, rounded up what the left-leaning Human Rights Commission said was a total of 19 persons, including half a dozen well-known leftist politicians.

Garcia, on his return, told inquiring diplomats that he knew nothing of the arrests and expressed anger. This led to speculation that unknown security officers, perhaps in collaboration with rightist civilians, had decided on the abductions on their own.

The defense minister on Monday, however, officially acknowledged that eight leftists had been rounded up on charges that they belong to the Revolutionary Democratic Front. He thus appeared to endorse the arrests, which had been described by a U.S. diplomat as "politically stupid" even if legally justifiable under El Salvador's state of siege.

"They are closing the way to a peaceful settlement," a leftist priest and political activist commented, charging that the abductions were a deliberate rightist attempt to frustrate any possible peace talks.

The guerrilla leadership, meanwhile, called in reporters Tuesday in Mexico City to make its peace proposals public before Magana had time to reply in private. Predictably, the Salvadoran right reacted immediately with angry denunciations, reducing any chance Magana may have had--however slim--to arrange something aceptable to left and right.

Roberto d'Aubuisson, president of El Salvador's constituent assembly and the right wing's most prominent leader, said it would be "the most vile treason and the most indescribable political absurdity for the government to sit and converse, much less negotiate, with those who have sown grief and destruction among the Salvadoran people.

"We will permit neither dialogue nor negotiations with the criminal groups" of the opposition, he added.

Against this background, Magana issued a statement Thursday emphasizing his refusal to negotiate with the guerrillas until they lay down their arms and insisting that "the essence of democracy resides in political dialogue through the electoral route."