On the first day of the attack, guerrillas surrounding the little town of Perquin used bullhorns to blast demands for surrender from the pine-covered mountainsides. People from the village said they felt like victims brought out to die in a stadium.

Then the attack began. For more than two days, 21 vastly outnumbered Salvadoran National Guardsmen and about an equal number of paramilitary patrulleros defended their little headquarters in the isolated town about 20 miles north of here, close to the Honduran border.

"I fired to the left, to the right, without pity," one young guardsman remembered. They were soon out of bullets. "We had a radio. We were screaming into it, 'Send us ammunition, send us food or get us out of here.' " No one came. Before dawn on the third day, those who could escape fled into the mountains around the village.

The force of Salvadoran guerrillas that entered Perquin on Aug. 10 held it until Aug. 19.

That single action, in one of the most remote corners of this country, marked the first significant step in a new stage of the Salvadoran war, and the first major success of a new leftist strategy that the Washington-backed Salvadoran government is ill-equipped to combat.

The Perquin offensive, the enormous economic and psychological impact of guerrilla sabotage against public utilities, and their increasingly successful effort to get countries such as Mexico and France to give political recognition to the guerrillas and their allies, have given the insurgents more momentum than they have shown or felt since their dismal failure in the January "final offensive."

Secretary of State Alexander M. Haig Jr. said Friday that in the face of this new guerrilla initiative the United States is considering "a whole array of political, economic and security-related measures" to halt the flow of Soviet arms to El Salvador.

Whatever their scale, the content of these shipments appears relatively unsophisticated. The guerrillas seem to have adequate small arms and ammunition at present, judging from the types of actions they carry out. But they still have no antiaircraft capability.

Current arms supplies, though vital over the long run, are much less important to the insurgents' present successes than their new-found tactical sophistication and overall coordination.

Their basic military strategy, as described by guerrilla spokesmen outside the country and as is perfectly evident here, is now a classically simple guerrilla war of attrition carefully tailored to Salvadoran realities. Gone are guerrilla dreams of a popular insurrection like that successfully led by Nicaragua's Sandinistas in 1979. The insurgents here tried that in January, only to discover that they had to fight alone, unaided by an uncertain and often resentful population.

To keep the Salvadoran military spread out and on the defensive, while wreaking havoc on the economy, the guerrillas have initiated a vast campaign of sabotage against public utilities.

More than 150 power pylons have been blown up over the last few months, leaving the eastern region of the country totally without electricity for weeks at a time and causing frequent blackouts even in the capital. Without electricity for their pumps, many of the towns in these areas also go without water. Recently the guerrillas have begun hitting the phone lines as well, and cities all over the country have been cut off sporadically from telephone communication for hours and days.

The overall damage is almost incalculable to an already crippled economy shrunk by more than 16 percent in the last three years -- certainly tens of millions, perhaps hundreds of millions of dollars.

The 18,000 to 20,000-man Salvadoran armed forces are clearly incapable of defending all 1,500 of the nation's pylons and, according to government officials, can no longer defend even the crews sent out to repair them.

Meanwhile, in one carefully selected region after another near their northern strongholds, the guerrillas make a push. First there was the area around Arcatao in Chalatenango Province early in the summer. When that operation ended, there was a pause, then Perquin here in Morazan.

Although the Salvadoran Army claimed the recapture of Perquin as a victory, interviews with witnesses, soldiers and other informed sources here, as well as with guerrilla spokesmen outside the country, suggest that the insurgents secured every immediate objective, tying up as many as 1,500 troops, inflicting casualties and generally wearing the Army down.

The initial fighting was heavy. The guerrillas had come in force, although the estimate of the guardsmen there that 500 insurgents were involved is probably overstated. When government soldiers abandoned the town early Aug. 12 the guerrillas were able to take at least four of them prisoner, along with more than a dozen patrulleros. Another three guardsmen were killed.

The number of guerrilla casualties is unclear, although one commander is known to have been killed.

Perquin, though small, was the first military headquarters ever captured by the guerrillas. But the taking of prisoners was another almost unprecedented move.

In itself the town was not significant, with a population of about 3,500, most of whom fled when the fighting began. But the guerrillas had put the Army in a position where it had to be retaken, and they made the military pay for every inch of road. This in an area where the Army has periodically claimed to have "cleaned out" the guerrillas.

Meanwhile, according to a senior government official, the Army was fearful that this regional capital was the guerrillas' real objective and decided it could not afford to deploy the necessary 500 soldiers from here.

The only way to get troops to the scene was by helicopter, but for at least a day none of the 10 U.S.-supplied Hueys was operating because of damage done by guerrilla ground fire and a lack of spare parts. damage was done is difficult to estimate since no foreign reporters have been allowed to the scene.

While the Army struggled to retake Perquin, the insurgents recaptured -- and still hold -- an important base camp near the village of Guacamayo that they had lost to the government in May.

And when government troops finally did reenter Perquin, according to informed sources here, the guerrillas were gone, taking their prisoners with them.

Guerrilla claims that some Honduran troops participated in the recapture have not been substantiated.

The Salvadoran military is tremendously reluctant to release casualty figures. Last month U.S. Ambassador Deane Hinton released statistics showing 1,300 government casualties since January, including more than 350 killed.

The apparent hope of the guerrillas is to be able to launch another Perquin-type operation soon, keeping the Army on the run from one scene of heavy fighting to another. This has the advantage, according to leftist sources in Mexico, of spreading responsibility for the fighting among the four often jealously competitive guerrilla factions. Perquin was entirely carried out by the Popular Revolutionary Army. A new operation in Chalatenango, for instance, would be the responsibility of the Popular Liberation Forces.

All these operations will be conducted on a small enough scale to avoid provoking Honduras or Guatemala into sending troops, one guerrilla spokesman said.

A buildup of guerrilla forces has already begun in the northwest region of Metapan near the Guatemalan border to slow down any Guatemalan incursion, according to the spokesman. The buildup is confirmed by U.S. intelligence sources.

But the spokesman added that the Mexican-French declaration, by recognizing the legitimacy of the guerrillas as a political force and warning against outside intervention may allow them to step up the size of their operations.

Meanwhile here in Morazan, peasants who once ignored the guerrillas, believing they had little chance of success, were impressed even by the action of the January offensive and, according to people who travel frequently in the region, were further inspired by the Perquin operation.