The U.S. Embassy in El Salvador has become a fortress waiting for attack. Iron gates and newly installed, thick concrete walls guard the main entrances; sandbagged gun emplacements are strategically located on the roof. Deep lines of stress mark the faces of the men and women who work inside.

Only two weeks ago, the primary fear of U.S. officials here was a possible assault on the embassy by Marxist guerrillas or an occupation by leftist demonstrators. One such occupation already had been attempted last October before being repulsed with tear gas.

In recent days, however, as the United States has threatened harsh political and economic sanctions to avert a rightist military coup backed by the country's ultraconservative economic elite, U.S. officials now believe that they have been targeted by right-wind extremists as well.

By continuing to prop up what is widely considered to be an ineffectual coalition of Christian Democratic policians and professedly centrist military men in this violently torn nation, the United States has found itself -- like the government it supports -- caught between two increasingly powerful extremist forces that are currently focusing most of their attacks on those in the middle.

Although the United States has borne the brunt of leftist enmity in Latin America for years, the new development in El Salvador is the transformation of its traditional ally, the right, into one of its most bitterly and dangerously antogonistic opponents.

Last Thursday, a spokesman for the recently formed Broad National Front, a conservative group claiming the support of landowners, business men and professionals, denounced Acting Charge d'Affaires James Cheek on television. Cheek, a deputy assistant secretary of state, has been running the U.S. Embassy since the previous ambassador, Frank Devine, demanded to be removed earlier this month.

Confirmation of the new ambassador, Robert White, has been delayed in Washington by conservative senators led by Jesse Helm (R-N.C.), who believe that White, a career diplomat who has served throughout Latin America and most recently as ambassador to Paraguay, is too liberal.

The Broad National Front spokesman accused Cheek and the United States of condemning El Salvador to communist totalitarianism by attempting to placate the left and supporting the Christian Democrats in the current junta.

Although the Front has denied any involvement, similar televised accusations have been followed by chillingly methodic murders of the people named.

Mario Zamora Rivas, a 38-year-old Christian Democratic member of the current government, who as solicitor for the poor handled the placement of orphans and other services for the destitute, was denounced by the Front early last week as a supporter of a Marxist guerrilla group.

On Wednesday, four of his employes were driving home to lunch when armed men forced their car to stop and blasted through the windshield with their machine guns. Two women in the car were seriously wounded. The two men, with most of the bullets aimed at their faces, were killed.

The terrorists chalked the sign of a skull on a wall behind the car and splattered gore around it as a warning. Afterwards, scores of people milled about the scene, staring at the bodies. Little children climbed trees to get a glimpse. The shooting must have been a mistake, friends of the victims said at the scene. The dead men were not political, they said.

Early Saturday morning the gunmen went after the Zamora Rivas directly, breaking into his house while he was giving a party. One of the Christian Democratic members of the junta had left only a short time before. The guests were forced to lie down on the floor and each was asked his name.

"We don't want any mistake," one of the gunmen reportedly said.

When Zamora Rivas identified himself, he was taken to a bathroom in another part of the house and shot 10 times before he died.

Left-wing terrorists have been capable of similar actions. They have attacked military outposts, shooting lone policemen on the streets, briefly taking over villages and executing officials or presumed informers after short public trials.

Numerous businessmen, both local and foreign, have been kidnaped by guerrillas and either ransomed for enormous sums or killed. South African Ambassador Archibald Gardner Dunn has been held since last November by guerrillas demanding a ransom of $20 million.

Spokesmen for the Front and other conservative groups say that right-wing violence is only a reaction to the alleged atrocities of the left.

"We are not ultratired," said retired Maj. Robert d'Aubisson, a former member of El Salvador's intelligence and counterinsurgency forces under the previous military government, who denounced Zamora Rivas on behalf of the National Front.

People in the streets of San Salvador, the nation's capital, say much the same thing. But it is clear that they are tired of the violence and the unrest generated by both sides.

There are enough problems without the violence. El Salvador -- a rural society -- is the most densely populated country in the continental Americas -- 5 million people in a nation the size of Massachusetts. There is staggering unemployment, especially for young people. In the high volcanic hills of the countryside, those who do work often earn no more than a dollar or two a day.

The money and the power have been in the hands of so few people for such a long time -- ever since 30,000 people were killed when the hemisphere's first communist revolution was crushed 48 years ago -- that democratic traditions were virtually unknown.

Salvadorans now find themselves living in a country that no longer seems to work, where the routines of life are interrupted constantly and hope for the future smashed by strikes, violence and political persecution.

"Look," said a middle-aged cab driver, "most of the people don't have anything to do with the ultraright, the ultraleft or the centrists. They just want to follow a straight path, earn some money and have some say in their lives.

"They want change but they don't want communism; they don't want civil war. They want to see the end of the oligarchy, but they hope they will just go away to their money in Switzerland or Miami."

But the powerful vested interests of El Salvador most of whose fortunes are in long-held coffee-growing land and recently started small industries, have made it clear that while their families may leave the country they intend to stay and fight and, if possible, either force the Christian Democrats out of the government or seize power directly.

At a meeting of influential coffee growers last week some of the richest men in the country made it clear that they would rather die than see the kinds of sweeping reforms that the United States and the current junta appear to advocate. "In 1932, the same situation was faced and 'conveniently' overcome," said one coffee grower referring to the earlier insurrection as he spoke to the assembly. "We must take a decisive stand."

That stand and the official position of the United States are now directly at odds. Few people in El Salvador are optimistic about a quick resolution of the new-found antagonisms, which is one reason why the United States says a right-wind coup, which it believes will almost certainly lead to civil war, has been only temporarily averted.