The buzzards are fat. They are the same color as the esplanade of gray and black volcanic rock that stretches 15 miles behind the San Salvador volcano, the sentinel overlooking El Salvador's capital.

At first sight it is as if the rocks had come alive, fluttering and bumping in patches among the burning garbage and broken bottles. Instead, it is the buzzards. The birds are busy, cleaning another skeleton.

This is El Playon, a lava field traversed by a major highway, whose edges normally are used as a garbage deposit.

Like many other such dumps, El Playon was converted into a clandestine dumping ground for bodies several weeks ago -- no one is certain just when. However, the size of the site makes it unique. There are so many bodies -- several dozen, perhaps as many as a hundred -- that no one is bothering to pick them up any more.

Reports of the body dump at El Playon began appearing in the local press as early as mid-September, but according to the Salvadoran Human Rights Commission, the legal authorities "no longer are bothering even to come and recognize the bodies, nor even arranging for their transfer to a morgue or burial."

In El Salvador's brutal civil struggle, the appearance of mutilated bodies long has become routine. The corpses usually are found at the dumps early in the morning, having been deposited overnight.

The Roman Catholic Church-related Human Rights Commission and the Legal Aid Office say that more than 10,000 persons -- not including soldiers or guerrillas killed in combat -- have been slain in El Salvador for political reasons in the last 10 months. They accuse the Salvadoran Army and police forces of being responsible for most of the killings. Army spokesmen blame the guerrilla opposition for the violence. Both sides concede that often there is no convincing proof for their claims.

A crucial point in U.S. policy toward El Salvador has been whether or not the Salvadoran civilian-military junta is capable of bringing the random violence under control. Discoveries such as El Playon would indicate that the level of noncombat violence in El Salvador is pretty much at the same level it has been at throughout the year. The Human Rights Commission says about 700 civilians have been killed in each of the last two months, which is the average monthly figure for 1981.

The Army and the president of the junta, Jose Napoleon Duarte, counter with charges that both the commission and the legal aid office are leftist fronts and that they manipulate fact and figures to win sympathy for the guerrillas fighting to overthrow the government since early this year.

While nonmilitary violence continues unabated in this tiny country, the leftist opposition seems to have broken the military deadlock and is making strong headway in the northern and eastern regions of the country. Furthermore, the Christian Democrat-dominated civilian component of the junta is being challenged by five right-wing parties that are calling for the resignation of the government before next year's general assembly elections.

The discussion of who is responsible for the violence is loud and acrimonious, but in the sweltering horror of El Playon there is only nerve-shattering silence, broken by the occasional scuttle of a lizard across the piles of garbage among the rocks. The real victims of El Salvador's war are those civilians whose bodies, like those of beached fish, appear here on the surface of the lava sea every morning.

A fresh body lies directly on the highway shoulder and by 11 a.m. the buzzards have made good progress. About 10 yards farther in, a jumble of bones lies beside the clearly identifiable bodies of a man and a woman. As a cluster of reporters approaches, the birds flap lazily away.

A road has been carved from the highway into the lava field. Bloodied clothing shows that others have walked this way recently. First there is a pair of trousers, then a shirt, then a T-shirt and then a small field the size of a baseball diamond, where femurs, pelvic bones and shoulder blades are strewn about as casually as the hundreds of discarded bottles of "Tic Tac," the local firewater. A reporter and a photographer find 30 skulls lying next to groups of relatively intact skeletons, then give up counting.

The skeletons are everywhere. The skulls generally show a full set of teeth, a likely indication that the victims were young. In El Salvador, where malnutrition is widespread, a 40-year-old peasant frequently will have only a few teeth left. The modest clothing lying about is all civilian, and there is a fair proportion of skirts, blouses and women's work shoes.

There are small cans punctured by bullets and upended bottles placed on the waist-high rocks, which makes it appear that the site also is used for shooting practice. More mysterious is a pile of shattered bones several feet across. The bone fragments, none more than a few inches long, are bleached to a calcium white, as if incinerated. A few feet away, there is a cardboard casing for a mortar shell. Another lies farther away.

A local photographer who visited the site last week said he ran into a woman who had found her brother's clothing and given it symbolic burial. "There were so many skulls, she couldn't tell which one to bury," he recalled.

The chief spokesman for the armed forces, Col. Alfonzo Cotto, said it was impossible for the Army to control the area of El Playon or to investigate the burial dumps. "There are simply not enough of us," he said. "Even the FBI and Interpol, whom we asked to help us investigate the murder of the four American church workers, were unable to come up with any evidence more convincing than a spent G3 cartridge, which is used by both the Army and the guerrillas. It's very hard."