Although El Salvador's rightist government has promised free elections for the first time in years next March, many informed observers here believe the government may not last until then.

A number of political and diplomatic analysts feel general armed rebellion could erupt in this poor, overcrowded central American nation within two to three months.

Beleaguered President Carlos Humberto Romero recently announced moves intended to defuse opposition, including readmission of all political exiles and international supervision of the March congressional and municipal elections.

Last week, Romero, an Army general, stunned members of his own military-based Party of National Conciliation by saying that, after more than 45 years of military presidents, the party's 1982 candidate may be a civilian.

Gen. Romero also invited the international and local Red Cross to inspect the prisons to disprove charges by the opposition and the Catholic Church that scores of political prisoners are being held secretly.

But the consensus among moderate and leftist opposition groups, as well as within diplomatic and some military circles, is that these dirst glimmers of what Lating Americans call a "salida" -- an "exit" from authoritarian rule into democracy, are probably too little, too late, and do not address the social and economic inequities from which El Salvador's problems utimately stem.

While El Salvador for years has suffered both government repression and left- and right-wing extremism, incidents of terrorism and labor strife have reached fever pitch.

Two priests have been murdered in the past two months, allegedly by right-wing terrorists who charged the church with subversion. Six clergymen have been assassinated since Romero's inauguration in 1977. The Aug. 4 shooting of the Rev. Arlirio Napoleon Macias as he was saying mass in the San Esteban Catarina parish was followed within days by the town's government-allied mayor.

In the small town of Armenia, 30 miles northwest of here, the mayor, a member of the ruling party, was murdered in April. Last week, his replacement also was assassinated.

Dozens of priests and nuns are on a week-long hunger strike at a downtown church to protest divisions among the country's six bishops and the failure of the majority to denounce both the murders of priests and, by implication, the government.

Other churches in at least three of El Salvador's principal cities are occupied by protesting members of the United Front for Popular Action and the Popular Revolutionary Bloc, peasant-based federations of militant groups. Following the internationally condemned government shooting of at least 80 persons during and following similar protests in May, Romero has said the security forces will not allow themselves to be "provoked."

But tempers are wearing thin.

Similar government promises have been made concerning militant labor strikes underway at four major factories here.

The strikes reportedly are organized by a committee associated with the Popular Revolutionary Bloc. The strikers use similar tactics of taking over industrial plants and holding executives or other workers hostage. At one clothing assembly plant where workers are demanding a 100 percent wage increase, American general manager William Boorstein has now been held captive for more than a week.

[On Wednesday, news services quoted officials as reporting, three members of the Bloc were killed in a clash with national guardsmen on a highway north of San Salvador.]

While the military has accused the popular groups of guerrilla-allied subversion, and there is a general feeling among moderate political groups that their members are sometimes manipulated by militant leftist ideologues, there is wide agreement outside the government that most of their demands are legitimate.

Moderate leaders and diplomatic sources are concerned that the scattered strikes and takeovers are test runs for a nationwide shutdown.

At the same time, a popular mutiny is feared. There is near panic within government, business and political groups that El Salvador's guerrillas will seek to imitate the recent revolution in Nicaragua next door.

Nicaragua's new Sandinista-led government has strongly stated it will not export its revolution, and there is no evidence that it has. But San Salvador is rife with rumors that, in the words of one leading politician, "there are a thousand guerrilla soldiers, armed and ready to go."

El Salvador's own guerrilla groups, three separate Marxist organizations whose numbers are wildly estimated at from 20 or 30 to several hundred members each, "have spared no effort to spread the word" that they intend a repeat of Nicaragua, according to one diplomatic observer.

"They talk about their 'seasoned veterans' coming back from the war there," the observer said, "and loans given the Sandinistas being repaid [with soldiers.] They talk about executing war criminals in the streets by the end of September.

While conditions here are generally similar, they are vastly different in specifics from Nicaragua while it was under the rule of the Somoza family. The guerrilla groups are far more isolated, and less willing to compromise and accommodate moderates, than the Sandinistas.

Opposition in El Salvador is focused not on a man or a dynasty, but ona system of official corruption and oppression and the agriculture-based economy. The more than 90 percent of the people who work on, rather than own, farm land are effectively excluded from the money economy.

As in Nicaragua, the lack of political access and government intransigence toward substantive change have provided fertile ground for violent eruption.

There is broad agreement that El Salvador is a battleground between two extremes, with the vast majority of its 4.8 million people caught in the middle and daily becoming more radicalized. The level of agreement breaks down, however, on the questions of who the extremists are and what to do about them.

The government denies authorship or approval of right-wing terrorism, but it portrays itself as unable to control the extremists. It counters every charge about the right with a complaint about the left.

Still, it says it recognizes the need for change. "The armed forces are ready for democracy," a government spokesman said. "There are going to be free elections here in March."

To opposition politicians, the mechanics of a "free" election are meaningless if, as one politician said, "you can drop your ballot in the box, and it gets counted, but you can still get beaten up for going to a campaign rally."

Antonio Morales Erlich, vice president of the leading opposition Christian Democrats, is a longtime political exile who benefited from recent reforms by returning here from Costa Rica three weeks ago.

In a televised speech last night marking the strongest public opposition statement here since the 1976 elections, Morales Erlich said repeated electroal fraud and official corruption have led to the "total illegitimacy of the government, its repudiation by the people, and the destruction of our institutions."

The government, he charged, "says from now on they're going to be good. If they had said that two years ago when we asked them to, we'd be in a different situation now. Nobody believes what they say about liberty . . . The problem is now. It is not a long-term question of waiting until 1982" presidential elections, he said. "Thousands more people will be dead by then."

The Christian Democrats, along with sectors of the church and more liberal business sectors, have begun a series of meetings with labor, peasant and other mass groups in hopes of coming up with a plan to pressure the government from several directions.But the process is a slow one, and there is a feeling that time is running out.

One solution that has so far been only whispered about is Romero's replacement.

During a little-publicized visit here several weeks ago, sources said, U.S. Assistant Secretary of State for Latin American Viron P. Vaky expressed strong concern to Romero and suggested that the date of presidential elections be substantially advanced. Romero, the sources said, answered that such a move would violate the constitution.

Others speak of the establishment of a transitional civilian-military junta or even a military coup from the center to forestall the possibility that one may be coming from the far right.

"Some say Romero is the problem," one observer said, "but others say it wouldn't make any difference if he was gone. Most just say it's too late, too late for anything."