An assault on this town last night by leftist guerrillas left three paramilitary civil guards dead, seven soldiers wounded, an armored car up on blocks for repairs, and frightened civilians wandering the dirt streets this morning among crowds of government soldiers brought in as reinforcements.

Guerrilla losses in the 2 1/2-hour shootout were not known. This was just one of several small clashes around the country and typical of battles that occur several times a week as El Salvador's war becomes a chronic yet worsening conflict.

In the department of Usulutan to the southeast, guerrillas were reported today to have cut the Pan American Highway, although further details remained sketchy. Guerrillas have been stopping traffic, digging ditches through the asphalt and haranguing stalled travelers with long political speeches in that area for several days.

Meanwhile, the insurgent radio broadcasts announced this morning that guerrillas have withdrawn from the town of Corinto in the northeast Morazan region, and denounced as lies Salvadoran government claims that the insurgents massacred 150 or more people in the Chalatenango village of Nueva Trinidad. According to the broadcast, on a new guerrilla frequency called Radio Farabundo Marti, the operation at Nueva Trinidad was a strictly military operation, killing 20 paramilitary forces and 15 soldiers.

As the daily skirmishes multiply, the clandestine terror that has characterized El Salvador's civil war continues as well. At midday, a ragpicker wandering through the refuse littered across the craggy volcanic landscape of El Playon north of the capital pointed out a few bullet-perforated, sunbleached skulls and spinal columns. He said they had arrived since he was last there two weeks ago.

As is often the case, he said he had no proof of who dumped the bodies, but like many Salvadorans, he assumed he knew. "The Army of course," he said, and noted that many were thrown on the rocks of El Playon at night during the curfew when nobody went out except government forces.

The local newspaper Diario de Hoy published a message allegedly received from the Maximiliano Hernandez Martinez death squad disassociating itself from a criminal gang led by an ex-Army officer.

The alleged death-squad spokesman said the leader of that gang "is corrupt and that is outside our ideals." The spokesman said the gang leader's claims to be part of a death squad were being investigated. The voice over the telephone also said that the majority of bodies at El Playon were results of his group's cleanup operations against subversives and delinquents, and that the death squad is preparing a new offensive.

Top government officials in San Salvador, meanwhile, have issued a new account of a massacre last Sunday that witnesses attributed to uniformed government troops in the capital's slum of San Antonio Abad.

Both civilian President Jose Napoleon Duarte and Defense Minister Jose Guillermo Garcia have offered reporters the new version that at least 19 people killed in that action were attending a meeting after midnight and fired on troops who investigated.

The official Army communique issued Monday failed to mention any such circumstances, saying the military was successful in rooting out various subversive "safe houses."

Residents of the area--including a teen-ager who said she and her two young sisters were raped after their brother was taken out and shot--told of being awakened and interrogated during the hours just after midnight.

The bodies of the dead brought to the city morgue the next morning were in many cases half-clothed and several showed bullet wounds at the heart or back of the head.

But the residents of this little town, about a dozen miles north of the capital, saw another side of the war last night.

Leftist guerrillas apparently are carrying a out a campaign of systematic attacks to eliminate small government emplacements. Shortly after 9 last night, some of the townspeople heard someone shouting that Tonacatepeque was surrounded and everyone should surrender.

The approximately 30 paramilitary guards decided to call for reinforcements rather than surrender. At about 10:30, a regular military patrol arrived and passed around town without incident, residents said.

There was no firing until the patrol started to assemble in the central square. Then the guerrillas opened up with automatic rifles and the fighting went on for more than two hours. Residents said three guards were killed and seven soldiers wounded. The military had no comment.

The arrival of new government reinforcements during the night caused the guerrillas to pull out, leaving behind the now-familiar refuse of this war--abandoned insurgent fortifications that were thrown up around the town, an exploded homemade mine used to ambush reinforcements, a trench dug through the highway nearby, the twisted bodies of the dead paramilitary guards--but no visible guerrilla casualties.

"They attacked from all sides of the city," said a 20-year-old guard in a yellow T-shirt, his old government-issue Garand rifle slung over his shoulder with a piece of bailing twine. The young man estimated the strength of the guerrillas as 200 to 300. Then he added, "I really don't know how many subversives there were." He looked blankly past one of the corpses on the sidewalk. "There were enough."