A week after the beginning of the leftist guerrilla "final Offensive" launching El Salvador's civil war, the government says the nation is under "complete control." But this town, an hours's drive from the capital, is one of several points in the countryside where the guerrillas roam unmolested.

"It's a day's walk across the area where we have no problem with the guard [the Salvadoran National Guard] or the police," said a 20-year-old peasant who has lived with perhaps 100 other guerrillas for the last three months in the otherwise virtually deserted town.

Government troops, with plenty to occupy them in the urban areas, have found little time or inclination to look in on villages such as San Lorenzo.

In the guerrillas' command station, another young man identifies himself as "Gustavo" and says he is "responsible" for the zone.

"We don't call this area 'liberated' because we aren't really able to administer it," Gustavo explains. "It's just that we are in control here."

The events leading up to the town's desertion began with the killing of an Army sergeant by a guerrilla group more than three months ago. The National Guard, which had no quarters in San Lorenzo, entered the town the following day and took reprisals. After 23 civilians reportedly had been killed by the guard, the guerrillas, with some civilian participation, evicted the government forces.

According to those who have stayed behind, most of the population left a few weeks ago in anticipation of bombing and assaults by the military during the "final offensive."

Unlike most of his comrades-in-arms, Gustavo is from the city, a former university student.

"Our forces have been on the defensive for a very long time," he said. "When we went on the offensive that weakness showed. We must learn from the experiences of the past week."

The guerrilla town has a frontier air about it. On the deserted streets the youths gather in small groups to share a cigarette or play-act war. Four youngsters compare rifles and strut about half jokingly, half proudly.

A boy eagerly displays his "recuperated" submachine gun, which he insists he took in combat from a former member of the Nicaraguan National Guard.

"Here's the proof," he says, pointing to the weapon's registration mark. But he doesn't know how to read. The inscription says only, "Made in Belgium."

The guerrillas in San Lorenzo are part of the Farabundo Marti Liberation Front's standing army. They say they participated in the attacks on sizable nearby towns, including San Esteban Catarina, which was under siege last week for four days. This is one of the eastern and north central provinces where the guerrilla organizations are strongest.

Four of the five leftist groups making up the front participate in San Lorenzo's guerrilla column.

All the groups have concentrated their strategy on building a peasant base in the impoverished countryside. One result of that work is the freedom with which Gustavo and his troops now roam through an extensive area in a sugar cane-growing region, where wealthy families long maintained private armies.

An old woman, one of the 300 or so civilians still living in San Lorenzo, said she was too old to bother moving.