A guerrilla commander has told a group of American journalists brought to this rebel-controlled village near the capital of San Salvador that insurgents fighting to overthrow the Salvadoran government have taken the initiative in the civil war and intend to step up the pace of the fighting.

Alejandro Montenegro, appearing Thursday with five other regional battle commanders of the Farabundo Marti National Liberation Front in a stronghold less than 30 miles north of San Salvador, said, however, that the guerrilla forces' stepped-up activity was not aimed solely at disrupting national elections scheduled for March 28.

The Christian Democratic- and military-led junta has challenged the leftist guerrilla forces to compete in the elections for a constituent assembly and to end the civil war, but the rebels have refused.

The guerrillas invited and guided five American journalists, who traveled from San Salvador to visit the territory, to demonstrate their control of the area and their efforts to build what they called the "embryo" of the type of society they hope to have if they take power. The press conference was designed to emphasize the unity of the guerrilla factions under a coordinated command.

But despite the show of unity, there were signs that the Liberation Front still has a long way to go to overcome the lingering, potentially explosive frictions among the five groups, all of which have fighting units in this region.

Montenegro and the other rebel leaders told the reporters in a news conference on the slopes of the Guazapa Volcano that they expect to win the fight against the civilian-military government headed by President Jose Napoleon Duarte unless the United States intervenes directly in the fighting.

The Reagan administration has not publicly ruled out direct U.S. military involvement to prevent a leftist victory here, but it says it is not planning such involvement. The administration's stated policy is to increase economic and military aid to the Duarte government and to back next month's elections.

The guerrillas said they still hope for a political solution to the Salvadoran conflict, but they cited the U.S. decision to replace immediately helicopters destroyed by guerrillas at the Ilopango air base last month as "proof that it is the United States that is obliging us" to continue fighting.

Montenegro, who said he planned the commando assault on the air base, said, "We know that it is the Reagan administration on which the junta depends. The key is whether the Reagan administration will try to shut us off with more force of arms or with a political solution. We do not want more deaths."

The region around the Guazapa Volcano, where the guerrillas appear in complete control despite repeated Army efforts to dislodge them, is strategically important because of its closeness to the capital. Virtually surrounded by government troop emplacements, it is one of the most difficult guerrilla-controlled areas for visitors to enter.

Another rebel area, in Morazan Province in the northeast along the Honduran border, was visited in January by foreign journalists.

Estimates earlier this year quoted by U.S. intelligence sources indicated that government forces have lost control of at least a quarter of El Salvador's territory.

While the five guerrilla factions operating in this area cooperated to create a highly structured political organization among the approximately 5,000 remaining residents, the differences among them were apparent. Even the rifles carried by fighters are labeled with the initials of their faction.

Signs of division were also apparent at the news conference.

Montenegro, a founder of the Revolutionary People's Army (known as ERP for its Spanish initials) reviewed the guerrillas' successes this year without noting the spectacular destruction of the Golden Bridge, carried out by the Popular Liberation Front (FPL).

The FPL was not represented at the news conference, and reporters were told later that when guerrilla squads go out on offensive operations the FPL fighters insist on acting on their own.

All five factions espouse a Marxist-oriented ideology but are divided by issues that sometimes seem highly technical to outsiders and by personal rivalries.

A major difference between the FPL and the other groups, according to a guerrilla who uses the nom de guerre "Ernesto Dreyfus" and who belongs to the Armed Forces of National Resistance (FARN), is that the FPL leadership considers the revolutionary idea of "prolonged popular war" to mean extended fighting before the final taking of power, while the other groups believe the taking of power is only one element in a prolonged struggle to consolidate the revolution.

Yet most of the people living in the guerrilla-controlled region seem less concerned with the dogma at the top than the new-found, and remarkably effective organization that they have experienced recently.

In this area where sentries behind rock fortifications can shout obscenities back and forth with their government counterparts across narrow valleys, where 13-year-old messengers carry .38 caliber revolvers, and gray-haired women have been trained to handle automatic rifles, the main fact of life is the possibility of having to fight.

Virtually every house, the makeshift hospital, the map-making shop and the school have bomb shelters and secret hiding places.

The craters of 500-pound bombs dropped by the Salvadoran Air Force during earlier offensives have left deep scars on the hillside, and the alleged massacres of whole families have left other scars in the community.

Our journey to the guerrilla-controlled area began when we were dropped off by a driver on an isolated stretch of road to be met by men who were at first invisible among the branches along a nearby hilltop. They took shape silhouetted against the glow in the sky from the lights of San Salvador. We walked for nine hours, stopping first in the dusty yard of a peasant home, sleeping fitfully to the sound of children coughing beneath their thin blankets.

In the hours just before dawn, exhausted from treks up one side of a mountain and down the other, we rested for three hours in a guerrilla camp at a settlement called El Mangal. As we lay stretched out on the dirt we could hear one guerrilla patrol after another coming in after a night in the countryside. Only snatches of their whispered conversation were audible, full of words about killing and being killed.

In the morning these mysterious men turned out to be boys and farmers dressed in motley fatigues, athletic jerseys, baseball caps and berets.

The final leg of the journey brought us well up into the range of hills around Guazapa, where we looked down on the town of Suchitoto and government positions close enough to shout at from the burned-off hillside waiting to be farmed.

Even as the night march to get here and back to the capital suggested the strength the guerrillas have in their stealth and endurance, firsthand observation indicated that they did not have enough weapons to mount a major military push against the government.

At one point a group of guerrilla combatants was turned out to give the visiting journalists a display of their training exercises. Seven of the 20 soldiers were asked to stand on the sidelines because they did not have weapons.

The top commanders denied reports that they were receiving weapons from Cuba and Nicaragua even when told that high officials in Havana make no secret of major shipments to Salvadoran insurgents before the January 1981 offensive. But no matter how the guerrillas' rifles--mostly American M16s--came into their hands, the battered weapons appeared to have been obtained some time ago. At lower levels, at least, the soldiers using them appear to have been trained on the job.

Dulas, a bearded, 25-year-old guerrilla, carried a battered Chinese rocket-propelled grenade launcher into fire fights and ambushes.

He said he had the opportunity to fire only one round from it in practice before he had to use it in combat. Only four months after he first joined the ranks of the FARN, he was taking part in the bloody assault on Suchitoto in the failed January 1981 offensive.

"Now I have a little more experience," he said. "Since then we have been invaded seven times by the enemy. Thank God we are alive. Because they shoot like they mean it, with mortars, artillery, airplanes, everything."

Like many of the guerrillas, including several members of the sector's high command, Dulas comes from this region, and the incentive to protect his home combines with more general ideas about the need to end repression, exploitation and "imperialism."

He knows that the government forces are better armed and better trained than the rebels, but he suspects, from firsthand experience, that their morale is faltering and that his is getting better.

"When they first came to invade we were afraid," said Dulas. "But now we see that when we fight back the enemy starts running."

Under the new system of government established by the guerrillas, the Guazapa Sector of the Central Front has been broken down into six "subzones," each of which has a "political leader of the masses" who is in charge of the area, a production head who oversees farming, a militia chief who looks out for the training of civil forces, and a company commander responsible for the regular guerrillas based in the area.

There is also a new legal system built around "commissions of honor and justice" in each subzone. These tribunals of three persons decide both civil and most criminal cases after a public hearing. There are weekly assemblies and meetings in each community to discuss a wide range of questions, but with the exception of the judicial commissioners none of the local officials are elected.