San Augustin, El Salvador — ''Pedro,'' a leftist guerrilla, stood on the veranda overlooking the village square and, calmly brushed his teeth. He and dozens more guerrillas from the Farabundo Marti National Liberation Front seized this town in a rain of bullets Jan. 25. Five days later, Pedro (a nom de guerre) was still taking his ease in the plaza, with villagers milling peaceably around him.
It was the first time he had walked openly in a town with his M-16 rifle slung over his shoulder since a brief guerrilla push during the national elections last March.
''We made a qualitative leap in Usulutan,'' Pedro said of the rebel offensive in this southeastern province, his somewhat stilted political lingo contrasting with his ragged clothes. ''We've gone from simple harassment to taking towns.''
In fact, the January guerrilla offensive in three provinces - Usulutan, Chalatenango, and Morazan - sparked one of the most intense periods of fighting in the nation's civil war.
Neither the Army nor the guerrillas are any closer to a big win. But the guerrillas' new pressure on their foes has opened a volatile phase in which the Army seems at times to be on the defensive. Leftists opened their campaign Jan. 8; the Army's counterattack was launched Jan. 17, concentrating in Morazan.
The guerrillas have adopted some new battle techniques. In Morazan Province, they challenged the Army to face them across a fixed battle line at a river north of San Francisco Gotera, the provincial capital. Soldiers tied pink or blue ribbons on their rifles to avoid shooting their comrades.
The fighting in Morazan is a head-on collision between two armies that have both greatly improved their equipment and capacity in the last two years. And it is the bloodiest, most intensive fighting of the war.
More than 6,000 Army troops, including all three American-trained rapid reaction batallions, are committed in Morazan.
Defense Minister Jose Guillermo Garcia says the fighting marks a ''decisive moment'' in the war, now more than two years old. It also breaks a general deadlock in the war that set in after the elections.
A Western diplomat who was invited to observe the Army's Morazan operation said General Garcia had only a minimal role in planning the battle. There have been rumors that the defense minister would resign since Lt. Col. Sigifredo Ochoa Perez rebelled against him in January.
The Morazan operation, a high-ranking former officer says, is intended to make a political show of the unity and strength of the armed forces in the wake of the Ochoa rebellion.
The operation in that province seems to be going forward despite a lack of consensus in the armed forces about how to conduct the war there.
The commander of the operation, Col. Jaime Ernesto Flores of nearby San Miguel Province, a ''crack commander'' according to US Ambassador to El Salvador Deane R. Hinton, has organized a conventional sweep into the rocky Morazan terrain. He has used some new flanking moves and airlifted troops successfully behind guerrilla positions.
Another officer, Col. Juan Pablo Gonzalez of the Fifth Infantry Brigade, has a slightly different view of the fighting. ''The towns in northern Morazan are not important objectives. The guerrillas are just trying to draw us into their terrain,'' he says, echoing the views of General Garcia and American military advisers.
In Usulutan, the guerrillas opened up 11 days ago with what appeared to be tactics to divert the Army from Morazan. Now they appear to have consolidated, at least temporarily, a new region of control stretching across the heart of the crucial cotton- and coffee-growing province. Heavy fighting occurred here Monday , and there are reports that the guerrillas have shifted their focus to Usulutan from Morazan.
Guerrillas are occupying at least two villages in Usulutan and have taken control of a stretch of the main highway along the south coast, bombing two key bridges. The guerrillas claim that 40 soldiers guarding one of the bridges fled without firing a round Jan. 27.
Last Saturday, soldiers recently posted to Usulutan were checking cars.
Guerrillas have also moved boldly against targets in and around the nation's capital, San Salvador, attacking the main Army garrison Jan. 26, as well as three other military posts.
After the attack on the San Carlos garrison in San Salvador Jan. 26, a military spokesman denied any fighting had taken place, although the sound of heavy artillery could be heard over much of the capital.
The guerrillas are pressing the Army in Chalatenango Province, too. According to press reports, guerrillas control the major road to neighboring Honduras.
Accurate reports of the fighting in Morazan are hard to come by. The Army has closed the combat zone to the press. But Ambassador Hinton says the Army achieved at least one of its goals in its blitz in that region:
''We're coming to the end of one of the more intense periods of guerrilla activity,'' Mr. Hinton said Jan. 27. ''They've been on the offensive for a month or so, and they've done pretty well.
''Their objective in the last couple of weeks was to take the provincial capital of Morazan (San Francisco Gotera). It would have given them what they've sought - a controlled area with something of importance in it, not just small villages.''
The Army has been concerned that the guerrillas might try to establish a capital in a so-called ''liberated zone.''
''They were stopped cold'' in their drive toward that goal, Mr. Hinton said of the guerrillas.
In December, guerrillas crept to within three miles of the main town in Morazan, and once lobbed three mortar rounds into it. So far they have not tried a direct attack on the town.
In mid-January guerrillas blew a strategic bridge almost from under the feet of one batallion, momentarily stranding some troops in a northern guerrilla zone.
The guerrillas appear to be winning the propaganda war as far as this latest series of battles is concerned.
In San Augustin, they turned 12 prisoners of war over to the International Red Cross Saturday as part of their policy of respecting the lives of the captured soldiers.
Cadet Carlos Alberto Consuegra, who trained in Fort Bragg, N. C., last year, told startled reporters, ''They treated me very well. I have nothing to complain about. I thought they were going to abuse me if they got me, but now I think just the opposite of them.''
A guerrilla guard was standing nearby, but Mr. Consuegra said he did not feel his statement was coerced.
The departmental commander of Usulutan told reporters the Army retook San Augustin just after they had visited there.