Within a few days the Salvadoran guerrillas' high command will call for "a popular uprising" that the insurgents believe will make elections scheduled by the U.S.-backed Salvadoran government for March 28 impossible to carry out, according to one of the guerrilla front's top five commanders interviewed here.

The commander, known as Ferman Cienfuegos, said the Salvadoran guerrillas are going ahead with plans to increase greatly the level of their offensive before the election despite fears expressed by their friends in Nicaragua and Cuba that such a step could push the Salvadoran conflict to an uncontrollable level and provoke major retaliation by the Reagan administration.

Cienfuegos said the guerrilla commanders have decided that the offensive is a vital element in the policy he outlined of talking and fighting in order to win the current war.

In the interview, which was arranged yesterday evening by concerned high Nicaraguan officials, Cienfuegos said he fully supports the electoral process but not until after a negotiated end to the fighting.

The current government's plan for the election of a constituent assembly, an idea heavily backed by Washington, is "a plan that is alien to the people and the guerrilla front , and because of that we are going ahead with our proposal to negotiate--and with the war."

"With a fascist dictatorship," Cienfuegos said in reference to El Salvador's current governing coalition of military officers and Christian Democratic politicians, "the only way to get them to understand anything is with force. Sadly, the Reagan administration sustains this dictatorship."

The call for a massive uprising, which Cienfuegos said would be broadcast over the guerrillas clandestine Radio Venceremos soon after he returned to El Salvador, would mark a major escalation in what has already proved an effective guerrilla campaign to sabotage El Salvador's economic infrastructure and wear down its armed forces.

By March 28, if the current government goes ahead with election plans, Cienfuegos said, "there will be no public transport, in addition there will certainly be no electricity in the country, business will have to come to a stop, there will be no traffic on the highways and we will have encircled several of the nation's cities."

A guerrilla call for insurrection and a "final offensive" in January 1981 was a failure. But while that action did not present Ronald Reagan with an "irreversible military situation" in El Salvador before his inauguration, as Cienfuegos predicted at the time, it did dramatically change and expand the scope of the war.

The January 1981 offensive provoked the resumption of U.S. military aid to the Salvadoran government despite the government's record of human rights violations, and the Reagan administration, soon after coming into office, announced that the line would be drawn in El Salvador against Marxist revolutionary advances in this hemisphere. Since then U.S. military aid to the government has increased dramatically, with $80 million in U.S. military assistance being spent in El Salvador during the first four months of fiscal 1982.

Sandinista officials here said that the concern among Nicaragua's revolutionary leadership is that a major new guerrilla offensive will be too successful, coming at a time when the Salvadoran Army appears especially weak after a debilitating and largely unsuccessful campaign around the Guazapa Volcano.

"If the Salvadoran Army starts to fall apart, I hope the guerrillas rush in with oxygen and hot towels," said one senior Nicaraguan official.

Of concern to both the Nicaraguans and the guerrillas on the diplomatic side of the fight is the amount of prestige the Reagan administration has invested in El Salvador along with its money.

Cienfuegos said the guerrillas want to leave options open. He described a letter phrased with conspicuous moderation that he and the other four top commanders of the guerrilla front sent to Reagan as "the opportunity not just to make a gesture. We can say that when you are building peace there are no winners or losers.

"We believe that the people, if they win the war, win politically and militarily," he said.

The guerrillas of the Farabundo Marti National Liberation Front (FMLN) and their political allies in the Democratic Revolutionary Front (FDR) repeatedly have proposed during the last year that negotiations be started in order to end the war and violence that have cost the lives of more than 30,000 Salvadorans since the closing months of 1979.

The insurgents have said they are willing to negotiate without prior conditions in talks that would include the governing junta, the military and other interested sectors of Salvadoran society.

Cienfuegos said a number of Western governments might be willing to guarantee the fairness of such a negotiating process and mentioned Mexico, Venezuela and France as examples.

The guerrillas have made public their program for the kind of coalition government they would like to see emerge from the talks. Cienfuegos, a Marxist, said he would envision a mixed economy with a major role for private enterprise and "semi-state" businesses as well as government-owned concerns.

Cienfuegos also insisted that "our objective since the beginning of the war never was to annihilate the Army" and that the fate of the Salvadoran military obviously would be a key point in any talks.

But the current Salvadoran government--whose strongest elements are precisely those military officers the guerrillas want to remove from power--has consistently rejected any and all proposals for serious talks since the guerrillas began offering them after the January offensive last year. The Reagan administration has backed this posture and insisted that the political solution to the Salvadoran carnage lies in the upcoming elections.

The guerrillas have said from the start that they would have no part in this month's elections. Cienfuegos said that "without the FMLN and FDR participating there won't be a political solution."

The guerrillas have accompanied their negotiation proposals over the past few months with ever less thinly veiled threats of a greatly widened war if they are rejected.

The Reagan administration, meanwhile, has accused the Nicaraguan Sandinista government and Cuba of continuing to supply arms, advisers and training to the guerrillas--charges the two governments have denied.

Cienfuegos said that, regardless of worries he noted of both the Nicaraguans and Cuban President Fidel Castro that a widening of the Salvadoran conflict might lead to a regionalization of major fighting and intervention by the United States, "we have said it's our own decision" to go ahead with the new step in the offensive that began building Feb. 1.

Cienfuegos was quoted in a guerrilla leaflet here as saying that such spectacular guerrilla operations as the commando attack on the Salvadoran Air Force earlier this year have helped rebuild "the insurrectional spirit" of the Salvadoran people.

According to Cienfuegos the insurgents can now count on 6,000 well-armed and seasoned combatants plus 15,000 to 20,000 militia members with some level of military training. In addition, Cienfuegos said that in his country of about 5 million people, approximately 100,000 are "organized" by the guerrillas and committed to the revolution, while an equal number are working for the insurgents in less regimented ways.

The size of the Salvadoran armed forces is estimated at 23,000 soldiers and as many as 80,000 part-time paramilitary guards.

Cienfuegos estimated that 1 million Salvadorans are sympathetic to the insurgents and that another large sector of the population just wants peace.

The guerrilla commander said that because of such support the guerrillas would have nothing to fear from fair elections.

He denied the Reagan administration's contention that the guerrillas have put forward their negotiating proposal because of fear that the elections would demonstrate their weakness. He said the proposal to negotiate reflected "a realism in the face of the crucial crossroads humanity faces between peace and destruction."

At one point in the interview, while describing the various levels of guerrilla strength, Cienfuegos said that the guerrillas' organization allowed them to protect civilians in their major zones of operation by evacuating or hiding them during Army offensives.

Asked why this was not done at the settlement of El Mozote near a guerrilla camp in Morazan Province, where an Army sweep in December allegedly killed hundreds of civilians, Cienfuegos said that those who were killed had not been organized by the guerrillas and were not clearly committed to the insurgent cause and that, therefore, the guerrillas were powerless to evacuate them.