Confronting a larger and better equipped Army, the Salvadoran guerrilla movement has adopted a major shift in military tactics likely to prolong the five-year-old civil war here but at a lower level of conflict.

The change, which has taken place over the last eight months, reflects the guerrillas' inability to sustain frequent direct engagements with the Salvadoran military now that it has expanded to nearly 50,000 men and absorbed several hundred million dollars' worth of U.S. aid over the last two years.

Instead, rebel leaders and U.S. officials say, forces of the Farabundo Marti National Liberation Front have broken down into smaller squads assigned to emphasize ambush and sabotage in what the leaders have baptized a long-term war of attrition. Reliance on these tactics, although they long have been part of the war, marks a departure from emphasis in 1982 and 1983 on striking what were called "decisive blows" in large-unit attacks designed to break the Army's will and provoke its collapse within a relatively short time.

"In this context, we can say that there is a change in the type of war," said Commander Joaquin Villalobos of the front's largest group, the People's Revolutionary Army, in a recent radio broadcast. "A change has taken place in recent months. There are changes in tactics. There are also changes in the form. We can say we have entered a war of attrition."

Rebel leaders interviewed in Mexico and Nicaragua insisted that the shift should not be interpreted as failure of their 1982-83 large-unit tactics. But they acknowledged that increased patrols, better equipment and more men in the Salvadoran armed forces, backed by U.S. reconnaissance flights from Honduras, have made concentrating guerrilla forces for large-scale attacks increasingly difficult.

"We are not saying the war is rosy for us, that we are defeating the Army," said one. "The war is raw. It is hard."

Villalobos also indirectly acknowledged the change in guerrilla fortunes during recent months, saying the rebels had a tactical advantage over the Army in 1982, 1983 "and a part of 1984."

In addition, rebel leaders said, the battalion-size guerrilla movements of 1982 and 1983 created unforeseen problems among the population. The arrival in a village of a 400-man unit seeking food and shelter often strained local resources and occasionally led to abuses against the people, they acknowledged.

A recent high-level defector from the front's second-largest group, the Popular Liberation Forces, said guerrilla commanders also have encountered increasing difficulty in securing enough ammunition to mount the large attacks. The defector, Napoleon Romero Garcia, attributed the munitions supply problem mainly to increased Salvadoran Army patrols along the country's southern coastline, where he said supplies arrive from Nicaragua by small boats, and intensified U.S. surveillance by radar and other monitoring in the Gulf of Fonseca separating Nicaragua from El Salvador.

"There are a lot of difficulties, and in these terms one of the weaknesses is ammunition," said Romero. "There is not enough ammunition."

Romero, 35, a former member of his group's 25-member Central Committee, was called Castellanos as a leader of the guerrillas' San Salvador urban underground until his defection last month. He was made available to reporters by the Salvadoran government after three weeks in military custody.

Romero said he gave himself up because of doubts about the war and that he was not tortured to make him talk. His former guerrilla colleagues, however, said he was captured and forced by torture to follow government orders.

In any case, observers here have noted that since a devastating attack on the El Paraiso garrison at the end of 1983, guerrilla forces have made only one large-scale assault on a concentrated Army position. In that attack, on the Cerron Grande dam last summer, a swift reaction by helicopter-borne Army troops prevented the rebels from taking all their objectives.

In addition, Army troops under a new commander in the traditional rebel stronghold of Chalatenango province have increased their presence there markedly during the past year without major resistance from guerrilla forces. Guerrillas used to feel so secure in the town of La Palma, for example, that they once invited foreign journalists in advance to attend a rebel-sponsored dance. But since October, Army troops have maintained a constant presence in the town and now soldiers instead of rebels are dancing with the local girls.

In the new tactical approach, guerrilla forces have concentrated on mining roads and ambushing Army patrols, seeking to inflict casualties rather than take military positions, their leaders said. They also have renewed emphasis on blowing up telephone poles and assassinating government or Army leaders in the cities.

Military Judge Rodolfo Araujo was shot to death here Thursday in front of the Roman Catholic Asuncion College, United Press International reported. Araujo had been working on more than 100 cases of political prisoners charged with "crimes against the security of the state," officials said. No group took responsibility for the killing.

Romero, the defector, said guerrilla forces also are suffering from a shortage of recruits. This is so, he explained, because manpower in the guerrilla-held areas of Chalatenango and Morazan has been "exhausted" after five years of war, and youths in what he called "expansion zones" have not risen to the call.

Reporters talking with guerrilla troops along roadsides here have noted an absence of new recruits recently. A rebel column encountered last week in the village of Tejutepeque, for example, contained only one combatant who had joined in the past year, a baker's son who said he fled to the rebels after being arrested and abused by the Army.

His column commander, a youth called Oscar, said guerrillas have not tried to pressure young men from Tejutepeque to join their ranks for fear of provoking animosity among the often-attacked village's few remaining residents.

Manpower shortages lay behind a controversial decision by Villalobos to press-gang youths into guerrilla ranks last year in Morazan, Romero said. The forced recruitment, denounced by some other rebel leaders as an error, was abandoned after several months when a number of the shanghaied youths deserted.

Romero estimated total guerrilla strength at 5,000 armed men, with 3,000 more logistics and "service" troops. The 5,000 fighters, he said, include slightly more than 2,000 in Villalobos' People's Revolutionary Army, 1,200 in the Popular Liberation Forces and the rest divided among the Armed Forces of National Resistance, the Armed Forces of Liberation and the Central American Revolutionary Workers Party.

U.S. estimates, particularly those made to Congress during debates over administration aid proposals, have been significantly higher, with some reaching 12,000. Guerrilla leaders have refused to discuss troop strength but insisted in the recent interviews that it is still rising.

Rebel leaders denied a lack of enthusiasm for joining the guerrillas among youths in contested areas, which they also call "expansion zones." But they acknowledged the guerrilla leadership has decided to emphasize political work among the population in coming months in an effort to implant guerrilla influence outside areas of traditional guerrilla control.

Villalobos contended in his address that the war of attrition also means guerrilla activity in previously peaceful parts of the country, seeking to undermine the economy and sap U.S. will to continue financing the government and Army.

The political work also was seen as an effort to counter the influence of President Jose Napoleon Duarte's Christian Democratic Party, which with the presidency and a Legislative Assembly majority controls government patronage and, in some measure, U.S. aid distribution in Salvadoran towns and villages. This is one reason behind a wave of kidnapings of recently elected mayors in eastern El Salvador, the guerrilla leadership has said.

Militarily, the new emphasis on attrition marks at least implicit abandonment of the earlier goal of hitting the Army so hard it would develop cracks. It also means guerrilla leaders have decided their battle to overthrow the government is turning out to be longer than they expected two years ago.

"We are in no rush," Villalobos said in his March 23 speech on the guerrillas' official Radio Venceremos. "We can resist for as much time as necessary."

In some ways, his accent on long-term attrition also amounted to acceptance of the "prolonged popular war" advocated by the late rebel patriarch Salvador Cayetano Carpio, who long had argued that long-term political work and small-scale subversion would be more effective than increased armaments and large-scale military operations.