As a city bus passed the Catholic seminary in San Salvador recently, two men in peasant clothes lunged drunkenly at each other, brandishing their machetes. The bus passengers commented excitedly on the action as the driver stepped on the gas, leaving the two men to continue their battle, a commonplace scene in Central America.

In El Salvador, as elsewhere in Latin America, centuries of dictatorship, hunger, disease, illiteracy and repression have created a violent culture.

In the bloody civil war that has torn this small nation for two years, both the Marxist-oriented opposition Farabundo Marti National Liberation Front and the civilian-military government face the problem of controlling and channeling this heritage of violence for their own purposes, while increasing their popular support.

A recent visit by reporters to the guerrilla-controlled section of northeastern Morazan Province showed signs that the fractionalized rebel army, in that region at least, has learned a great deal about channeling this violence since the early days of the war, when the guerrillas had to scramble to gain control of their fanatics. The guerrillas stressed these changes to the reporters that they conducted on the tour.

Repeated requests by The Washington Post during the past six weeks to the government in San Salvador to travel with Army forces on patrol and to be taken to areas in which the government has reported guerrilla atrocities are still pending. The junta headed by Christian Democrat Jose Napoleon Duarte asserts that, like the guerrillas, it has begun to curb the violent excesses committed by forces under its control in this chaotic civil war.

According to government sympathizers living in refugee camps in the eastern part of the country during the early months of last year, many would-be guerrillas at first seemed to understand "the struggle" as an expanded version of family feuds.

These and other sources said that during the latter half of 1980, when the guerrillas moved from a strategy of protests on city streets to one of taking control of rural areas, inflamed peasants who recently had joined the guerrillas went on self-appointed "spy cleanups" and "took control" of villages, forcibly conscripting teen-agers into their bands.

Since the failure of their attempted "final offensive" in January 1981, the guerrilla organizations appear to be making the effort to control their violence-prone soldiers a central element of the campaign to win back popular support that their own violent excesses had cost them. The guerrillas' Radio Venceremos broadcasts lectures on the importance of respecting the civilian population and treating prisoners humanely. Internal documents spread the same message.

During the journalists' visit to guerrilla-held areas of Morazan Province in January, rebel leaders displayed prisoners taken in combat and allowed the reporters to interview them privately.

In these areas of Morazan, the problem of dealing with violent peasant traditions crops up frequently in conversations.

"I tell the compas comrades that a real revolutionary, a real man doesn't beat his wife," says Alvaro, the head of a guerrilla military school who, like other fighters, uses only his nom de guerre.

Alcohol and violence are closely linked in Central America, and drinking is forbidden in the guerrilla area this reporter visited. In a small camp high in the mountains, three men took turns with a pick, shovel and saw, building a deep, tree-trunk-covered air-raid shelter. A sign full of spelling mistakes said the three got drunk the night before. "Please do not slow them down by talking to them," the sign requested.

At a military camp in Morazan, a guerrilla called Melo, in charge of defense for the high command of the northeastern front, explained why the guerrillas have halted forced conscriptions: "We would make ourselves militarily vulnerable if we had people unwillingly with us: they could identify our location, our secrets, everything."

The guerrillas do not deny that they practice ajusticiamentos, a word that literally means "just execution" and is used to describe the killing of government spies and members of antiguerrilla paramilitary bands. In fact, the guerrillas usually publicly take responsibility for these assassinations, and there is no evidence that any guerrilla group has ended the practice of "executing" individuals it considers dangerous.

Many Salvadoran and foreign sources also have said that guerrillas have murdered the families of government spies and paramilitary forces.

Guerrilla leaders acknowledge that their efforts to train their forces to use violence in a disciplined way result from a practical need.

"We are nothing without popular support," said Alvaro, the head of a guerrilla military school in Morazan. "If one more person joins us today, that is one day less we have to fight the war."

Two years ago the guerrillas' front organizations could call 200,000 supporters out onto the streets of San Salvador on very short notice. A year later, when guerrilla forces began what they billed as the "final offensive" against the government, their call for a popular uprising in the cities went virtually unheeded.

Former U.S. ambassador to El Salvador Robert E. White said in an interview in Washington that excesses by the guerrilla forces during the early part of the war contributed to the drop in support last year.

"The situation has now changed substantially," he said. "The revolutionary movement went from . . . disparate, uncoordinated groups that massed people in the streets and funded themselves through kidnapings to a rural strategy." White said this change of strategy had diminished the amount of terrorist violence by the guerrillas.

In the late 1970s, a guerrilla faction called the Popular Liberation Forces kidnaped several businessmen, government officials and diplomats, including foreign minister Mauricio Borgonovo, who was killed by the guerrillas in 1977. Their civilian support group occupied several embassies and the national cathedral.

"The bombings of the houses of important people, the firing of antitank weapons into the U.S. Embas-sy . . . this sort of thing has diminished markedly," White said. The guerrillas are now "more disciplined, and have more organization," he added. "It is now a coordinated insurgent movement that controls important amounts of territory and can launch military attacks from safe havens."

In testimony Feb. 2 before the House inter-American affairs subcommittee, Thomas E. Enders, the assistant secretary of state for Latin America, said that the Salvadoran government has made progress toward controlling abuses by its troops, but "the FMLN boasted on Radio Venceremos that it inflicted more than 2,000 casualties in the last seven months of 1981."

He also testified that "there have been incidents in which noncombatants have suffered terribly--at the hands of guerrillas, rightist vigilantes, government forces, or some or all of them--and that the insurgents have also repeatedly fabricated or inflated alleged mass murders as a means of propaganda."

A U.S. official said that even though "not everything" reported by Radio Venceremos "is true or accurate, even if they are boasting you have to take it seriously."

The source said that while it is true that guerrilla forces are receiving more training, both in El Salvador and Cuba, he suspected that the decision to take prisoners rather than kill them was simply intended to gain favorable publicity.

"I don't believe the prisoners number more than a score," he said, adding that he believed most guerrilla prisoners are killed.

The U.S. source cited captured "internal self-criticism" documents that gave examples of looting and other abuses by rebel forces as evidence for his contention that guerrilla forces commit abuses.

The Salvadoran government and opponents of the guerrillas accuse the People's Liberation Forces, one of the five groups making up the Liberation Front, of being responsible for most of the abuses by the opposition insurgents. The People's Liberation Forces, which is strongest in the northwestern province of Chalatenango, is repeatedly described as a group of "fanatics."

Late last year the People's Liberation Forces invited several reporters to visit what they call their "control zones," but a guerrilla representative later called off the trip, saying, "military conditions did not guarantee the reporters' safety." John Snow, a British television journalist who visited the area late last year reported, however, that he found the guerrillas well-organized and disciplined.

Alex W. Drehsler of the San Diego Union also visited the People's Liberation Forces in Chalatenango early last year and quoted a guerrilla leader as saying their policy was to spare the lives of Army troops captured in combat, but to execute members of paramilitary groups and other government armed forces whom they consider "mercenaries." Drehsler also reported that a guerrilla was arrested for raping a village woman and he was told the man would be executed.

The Washington Post has not been able to gain access to the areas of Chalatenango Province where the greatest number of abuses by guerrillas are said to have occurred.

The People's Liberation Forces is the largest of the guerrilla groups and is believed to control important parts of Chalatenango and San Vicente provinces. It is also the most heavily influenced by the "theology of liberation" espoused by Roman Catholic activists in Central America.

The area of Morazan visited by this reporter is under the control of the other major guerrilla faction, the People's Revolutionary Army.

The major difference between the two groups is a disagreement about strategy, which may have been resolved. The People's Liberation Forces, led by an aging former baker, Salvador Cayetano Carpio, backs the strategy of "prolonged popular war," which can be loosely defined as extensive political organization, war of attrition and less concern with weapons and military strategy.

The People's Revolutionary Army generally backs a call for a national uprising in the towns and cities.

Both major guerrilla factions appear to be made up primarily of peasants, but the leaders of the People's Revolutionary Army appear to be former university students or graduates while the People's Liberation Forces leaders generally have a working-class background.

A third guerrilla group the, National Resistance Armed Forces, has units in the area around Guazapa Volcano near San Salvador. It is an offshoot of the People's Revolutionary Army with members from trade unions and the universities.

Two smaller groups without significant armed forces are the Communist Party of El Salvador, which provides the insurgent front with a link to the Soviet Bloc, and the Revolutionary Party of Central American Workers, led by Fabio Castillo, a member of a junta that ruled briefly in 1961.

Catholic activists have had an important role in the guerrillas' efforts to stop undisciplined violence by their followers.

The Rev. Roger Ponseele, a Belgian priest who has worked in El Salvador since 1970 and is now teaching and saying mass in the Morazan guerrilla zone, said the pervasive influence of the progressive Catholic Church on the revolutionary movement has been very helpful in reorienting the traditional peasant violence. He also thinks the peasant guerrillas are radically different from the government's peasant soldiers.

"The compas understand the importance of taking prisoners, for example, very well now. They know life has to be respected because we are creators, not destroyers," Ponseele said.

"The key has been to supply a motivation other than personal revenge. There is a man who plays the violin for the mass, for example. The Army killed his wife, his in-laws, his children, but he doesn't talk about killing anyone in return. He talks about the future of the revolution."

Supporters of the "theology of liberation" clearly have had a major role in building the revolutionary forces.

Nolvo, formerly a prosperous peasant, now a guerrilla commander, ponders a question about how he got his start in rebellion.

"It was the priest," he says. "He talked to us about inequality and oppression. I saw that it wasn't fair, what the Army did. I saw people suffered too much from being poor." The answer was typical. At a crucial point in the country's history, the guerrillas' goals coincided with the teachings of "liberation theology."

As a result of this kind of preaching, government soldiers cracked down on the priests and nuns working with the guerrillas and in peasant areas.

Again and again, motivation appears to be a major difference between the guerrillas and the government troops. The author of the "Do Not Disturb" sign for the punished guerrillas at the mountain camp was Jorge, a wry, musing man who learned to write a year and a half ago. He interrupted his duties as camp commander to discuss his own experiences in the government Army.

"We were taught to defend our homeland, but they never told us what homeland belonged to us. They taught us machismo. I felt very good with a gun and a uniform--very handsome, very hombre. But when we were told we might get a bullet here--pointing to his forehead--for wearing that uniform, we all decided we didn't look handsome enough."