Stretched high between two towering royal palms in the central square of this sun-baked market town, the red banner of the Farabundo Marti National Liberation Front and a proliferation of revolutionary graffiti on the bleached plaster walls welcomed visitors last week to the other El Salvador, the one that did not vote in the election that surprised and captivated much of the world.

While the electoral commission finished up the long and slow tabulation at midweek in the capital of San Salvador, a special guerrilla unit of 100 men was still locked in combat in the streets of nearby Usulutan, El Salvador's fourth-largest city, with most of the troops the government could concentrate in this region.

As these battles went on, a much larger guerrilla force of about 500 was fanning out into the rural villages of this important agricultural province of the same name.

Here in Santa Elena, located on the forested skirts of Usulutan Volcano, members of the larger force talked to the people, held political rallies to explain their cause, daubed the streets with inflammatory slogans, gathered food and medical supplies and recruited fresh supporters for their ranks--sometimes by force, more often than not by simple persuasion, residents said.

The government was able to reestablish its control of Usulutan city at midweek, sending heavily armed patrols through its littered streets after a sudden guerrilla withdrawal shortly before dawn Wednesday.

But the guerrillas' political campaign--conducted not only here in Usulutan Province but also, according to sources close to the movement, in similar rural outposts in other guerrilla strongholds such as the provinces of Morazan, Chalatenango, Cuscatlan, and San Miguel--was thorough, and, it would seem, fairly effective in some places.

While government officials and U.S. diplomats in the capital of San Salvador, 56 miles to the east, were basking last Sunday in the glow of the impresssive voter turnout of 1.3 million and pronouncing the election a major political defeat for the guerrillas, shows another aspect of the Salvadoran political picture.

The scene here suggests that there are at least two El Salvadors: the more visible one seen by policy-makers in the capital or interpreted for them by the local officials, often military, whom they rely on for information from the provinces; and the other hidden, sullen, carefully silent world of the countryside, where the guerrillas long have drawn their support.

Diplomats and intelligence sources suggest that this silent world of countryside that is either under guerrilla control or outside of government control accounts for one-quarter to one-third of El Salvador's land surface. Observers say they have no way to calculate what percentage of El Salvador's 4.5 million people live in this situation, although some informed sources have estimated that 100,000 live in guerrilla-controlled territory in Morazan Province alone.

Here, the official wisdom that the election has dealt a potentially fatal political defeat to the guerrillas is not immediately evident.

In a land where gratuitous violence has bred a natural reserve among the survivors, townspeople in a place like Santa Elena do not speak easily of their experiences to a stranger from afar. That is especially true when the stranger has driven into town alone along a deserted road still echoing with the sound of government artillery and helicopter-borne heavy machine-gun fire raining down on the village of Palo Galan five miles to the south. A guerrilla rear-guard unit was reported to have holed up there after the pullout from Usulutan city.

Expressed opinions here are cryptic. People are cautious about speaking openly more because of fear of what one's neighbor might overhear and report to authorities or the rebels than because of concern at communicating with a foreigner who, in the trauma of El Salvador, is considered the lesser threat. As a result, a whole new language, using nuances, winked eyes, knowing smiles, subtle hand or body language, fills in the unstatable.

In Santa Elena Tuesday and Wednesday, there was no sign of a government presence--no policemen, no civil defense workers, no soldiers.

"They ran away a week ago," ventured one middle-aged shopkeeper.

The local priest, standing in a doorway dressed in civilian clothes, said the town had been incommunicado for almost a week. He seemed unperturbed by the fact the red banner still hung in front of his church, but he asked imploringly that he not be quoted lest someone take offense at even the studiously noncommittal nature of his conversation.

The rebels who briefly occupied Santa Elena were members of the Farabundo Marti National Liberation Front, a coalition of five Marxist-led military forces that have been engaging in guerrilla warfare against the military-civilian government for the past 2 1/2 years. They are allied with a large number of left-of-center political groups, including peasant organizations, labor unions and middle-class political parties.

One resident of Santa Elena said that on the day of the national election, many people from the countryside had come into town to vote. They were frustrated, however, by the fact that no ballots or ballot boxes had been delivered from the provincial capital as a result of the fighting there that had begun two days earlier. He shrugged as if to say it does not matter.

"I would have voted if the ballots had come," he said softly, "but only because in small towns like ours one would be conspicuous if one did not vote. Here, that could be dangerous, even fatal."

Later on the day of the elections, the muchachos, or "boys" as the rebels are called by their sympathizers marched into town about 100 strong. According to about two dozen townspeople interviewed in the streets of Santa Elena, the guerrillas acted respectfully. There was no violence, no rancor, although a "tax" of 100 colones, or about $40, was collected from half a dozen of the town's more affluent citizens.

The guerrillas painted the walls with their revolutionary slogans--"Death to the Assassins" and "United We Will Triumph"--and called the town to assemble in the central square for a political lecture in which they denounced the election as irrelevant and urged town residents to support them in their struggle against the Army-backed government.

The young were urged to join the guerrillas. Asked if any of their group had gone off with the rebels, a group of teen-agers standing by the priest's door signaled with a tilting hand gesture that some had. There were no reports of forcible "conscriptions" here, but refugees from Palo Galan, where the guerrillas had established a major encampment during their attack on Usulutan, reported that some young people were forcibly drafted into the ranks of the guerrilla army.

Although their political speech was laced with Marxist rhetoric, the guerrillas were generally respectful of private property, only breaking into the pharmacy to take medicines and bandages.

After about four hours, they marched out of town, going south toward Santa Maria, another hamlet, for a similar rally.

Two days later, listening to the bombardment of Palo Galan, the knots of townspeople in the streets, their ranks swelled by refugees fleeing in increasing numbers from the south, were clearly worried that the government might inflict harsh reprisals on them if the slogans and banner were discovered.

The guarded sentiments that one hears in such isolated villages around Usulutan city resemble comments that can be heard in other areas where the guerrilla presence--and military retaliation--have been strongest, such as in Morazan Province in the northeast and Chalatenango Province due north of the national capital near the rugged Honduran border.

One apolitical Catholic priest said of the mood he sensed from his parishioners scattered throughout a province he had worked in for more than a decade: "The muchachos are getting stronger, not weaker. They are everywhere now, and even people who recoiled from the idea of their armed struggle a year ago are today giving them their quiet backing."

Even a military man like Col. Domingo Monterosa, the commander of the U.S.-trained Atlacatl Battalion, conceded that the guerrilla forces are stronger and more effective than ever today. Interviewed on the outskirts of Usulutan this week after his rapid strike force had been called in to reinforce the Usulutan garrison, he spoke respectfully of the new "military capacity" his opponents had displayed in recent months.

Quick to insist that the guerrillas are indeed isolated, Monterosa nevertheless admitted that the new effectiveness of the guerrillas is in part the result of certain local participation and support.

The guerrillas, he said, have better mobility in El Salvador's rugged volcanic terrain and, in cases such as their attack on Usulutan, have managed to move in large concentrations across a heavily populated countryside without the government getting reports of their movements.

Asked if this does not indicate a certain amount of local support, he said: "Yes, they do have many locals with them. But the support they have can only come from fear of the guerrillas, not out of respect."