Gen. Jose Guillermo Garcia, 49, long considered the most powerful man in the U.S.-backed government here, announced his resignation as defense minister today under heavy pressure from fellow officers who said they feared his leadership would result in their defeat by leftist guerrillas.

U.S. officials here and in Washington also have been privately critical of Garcia's recent performance. Although they have come to rely on him to support their reform programs, and he helped thwart rightist attempts to take over the government last year, American officials carefully distanced themselves from Garcia after a January mutiny by one of his most effective field commanders and a series of military advances by the rebels during February.

Last week a mutiny by the commander of the Air Force was barely averted when President Alvaro Magana finally agreed definitely to honor a commitment reportedly made in January to remove the defense minister.

Magana announced this afternoon that Garcia's replacement, subject to approval by the constituent assembly, is Gen. Carlos Eugenio Vides Casanova, 44. The new defense minister, the only other general on active duty in the Salvadoran Army, is close to the president and for the most part respected by the military establishment.

Some officers have expressed reservations during the past few months that Vides Casanova is too close a friend and ally of Garcia. Also, like Garcia, he is without combat experience in the current civil war. Some officers who had pushed for Garcia's removal said last week they were concerned that Vides Casanova will make the same strategic and organizational mistakes that they believe his predecessor made.

Vides Casanova also has the serious political liability, insofar as U.S. support is concerned, of having commanded the National Guard since 1979. Its members are accused of some of this nation's most publicized human rights abuses, including the 1980 murder of four American churchwomen.

Since Vides Casanova's name emerged earlier this year as Garcia's most likely successor, U.S. officials have taken pains to say that he was helpful in efforts to clean up the National Guard and that he was particularly cooperative in the investigation of the murders of the churchwomen. Five national guardsmen are under arrest for that crime.

But U.S. diplomats who were serving here at the time that case was broken in the spring of 1981 said recently that Vides Casanova was a constant obstacle to the investigation and afforded American and Salvadoran investigators virtually no help in finding the killers.

"He stonewalled," said one of the U.S. diplomats closely involved with the investigation.

In Garcia's farewell at a press conference this afternoon he concluded by saying, "Every beginning has an end, but every ending is a beginning."

But it is not clear yet if Garcia's departure as presented today will resolve the problems associated with his almost 3 1/2 years as the decisive power in El Salvador's series of juntas and its current transitional government.

Garcia was frequently accused of placing his political allies in top commands, both at the ministry and in the field. He said today that aside from his own replacement there have been no other changes in the high command and he implied that there would be none.

Having driven the most liberal members of the officer corps into insignificant posts or out of the country during a series of political showdowns during 1980, Garcia was left as virtually the only man to whom Washington could turn for decisive support on questions of reform, human rights and elections which are vitally linked to U.S. support for this government.

Many of the senior officers who pressed hardest for Garcia's ouster have reputations as some of their institutions' most politically conservative men. A few, like Lt. Col. Sigifredo Ochoa who led the January rebellion and is now attached to the Salvadoran Embassy in Washington, have longstanding personal ties to rightist leader and constituent assembly president Roberto D'Aubuisson.

When D'Aubuisson put together a coalition after last year's March 28 elections and almost made himself president of the country, Garcia intervened and engineered the appointment of economist Magana to the post instead.

Today Garcia portrayed himself as a key figure in helping to build democracy here and acknowledged only in a broad sense the turmoil and tragedies in which he has been intimately involved since he took over the Defense Ministry on Oct. 16, 1979, one day after a coup led by junior officers espousing social and economic change.

As Washington has tried since then to keep leftist revolution from sweeping this country with a history of inequities and repression, Garcia became the symbol of its most powerful, troubling and troubled institution.

According to El Salvador's Catholic church more than 30,000 Salvadoran noncombatants have been killed in political violence since Garcia took command of the armed forces, and most of the attributable deaths were at the hands of men under his jurisdiction.

Garcia did not directly address that question today except to reiterate his often-stated support for human rights.

In response to a question about failings in his administration, Garcia said, "The only one who doesn't make mistakes is who? God. All humans make mistakes."

Looking composed but hot under the television lights in a tightly packed conference room at the Defense Ministry, Garcia said, "There is no rancor, there is no resentment" in the departure.

Garcia's rise to prominence in the eyes of his peers and eventually in the nation as a whole began as long ago as 1956 when he was sergeant of cadets in the Gerardo Barrios Military Academy, a position roughly equivalent to class president in the institution from which all of this country's top officers and most of its presidents have emerged since the 1940s.

Much of Garcia's career in the 1960s and early 1970s was spent teaching or in administrative jobs at the academy, where he formed close relationships with many of the officers he would later command and who would support his rise to power.

By 1974, as second in command of the presidential staff Garcia was looked upon within the military as possible presidential material. But too young and by some accounts too liberal in the eyes of his superiors he was put into a potentiallly lucrative and influential position as head of the state telephone and communications system, a job intimately tied to intelligence gathering.

By the time of the 1979 coup, Garcia was the commander of a brigade in the rich central province of San Vicente.

The more liberal young officers and their leftist civilian allies involved with the coup doubted Garcia's interest in reforms and were suspicious of his human rights record. Despite strong protests they initially accepted him as defense minister at the demand of conservative officers whose support was necessary to the coup.

By the end of December 1980, Garcia was able to force the resignation of every civilian member of the junta and the Cabinet. In 1981 the Christian Democratic Party, which had joined the government, divided in the face of continued massacres by Salvadoran soldiers, and only the more conservative faction continued to participate in the government.

By November of 1980 Garcia had effectively neutralized the "military youth" led by junta member Col. Adolfo Arnoldo Majano, and in December Majano was forced out of the government and into exile.

In that troubled period just before the guerrillas launched their January offensive, members of the Salvadoran armed forces allegedly murdered at least six and possibly seven American citizens, causing the suspension of aid by the Carter administration.

At the end of November 1980 virtually the entire political leadership of the left was abducted and executed in the middle of the capital with the alleged collusion of Salvadoran security forces. Garcia consistently claimed to know nothing of the crimes and to lack sufficient control over his soldiers to avoid them.

The Salvadoran Army was, however, able to weather the January offensive, with U.S. emergency aid. The Reagan administration then renewed and massively increased both economic and military assistance.

As he leaves, Garcia's military is being almost totally transformed--equipped, trained and advised--by the United States.

One of the most consistent criticisms of the outgoing defense minister and his allies is that they were unable or unwilling to use the kind of counterinsurgency techniques advocated by Washington. These include the deployment of small units to fight the guerrillas on their own ground in their own style.

Under Garcia the tendency of the Salvadoran Army has been to make massive sweeps, employing thousands of soldiers to "clean out" guerrilla strongholds, then pulling out.

As one Salvadoran major trained by the United States said last week, "How many times have we fought for San Vicente province? Twenty times? How many times are we fighting for Morazan? How many times for Guazapa? You have three or four battalions go into an area. You die for that area. You clear that area and you hold it for 15 days then you give the troops a vacation and give that area back to the guerrillas."