A leading Salvadoran Army field commander mutinied today, cracking the unity of the U.S.-backed armed forces fighting leftist guerrillas in a three-year-old civil war. He urged fellow officers to help him throw out the defense minister, who wants to consign him to "diplomatic exile."

The rebellion, by Lt. Col. Sigifredo Ochoa Perez in rugged Cabanas province, brought to the surface long-simmering tensions pitting Defense Minister Jose Guillermo Garcia against some of his officer corps, particularly Ochoa, and leaders of the country's extreme right, particularly Constituent Assembly leader Roberto D'Aubuisson.

It highlighted difficulties faced by the United States in trying to shape the 20,000-man Army into a fighting force capable of crushing the guerrilla movement despite political pressures here that impede its effectiveness and diplomatic pressures from Washington that threaten an aid cutback because of human rights abuses.

The Army's top officers gave no sign of following Ochoa into mutiny today, despite reported statements from some junior officers that they would refuse orders to move against Ochoa's 600-man battalion, headquartered in the Cabanas provincial capital of Sensuntepeque about 45 miles northeast of San Salvador. As a result, no violence was reported and the high command in the capital was in contact with Ochoa in an attempt to talk him out of his revolt.

The rebellion nevertheless was interpreted as a serious setback in the fight against the 4,000-man guerrilla force of the Farabundo Marti National Liberation Front. Ochoa, 40, has often been cited by leaders of the 55-man U.S. military advisory group here as an example of the kind of aggressive, corruption-free officers that they are trying to encourage for key commands in the Salvadoran Army.

In Sensuntepeque, Col. Ochoa told special correspondent Chris Hedges, "Garcia has betrayed the Army and turned the Defense Ministry into a political operation. He is a corrupt man who keeps his family in Miami and does not care about El Salvador, only his own gang."

Ochoa said several times that he was not instigating a coup. "I am a soldier who obeys his commanders," he said, "but I must fight against a man who is practically a dictator." Ochoa said he believes in democracy and hopes to further democratic institutions in the country.

In a separate development, the leftist guerrillas' Radio Venceremos said the rebellion was "nothing more than a fight between assassins" of the "fascist high command," United Press International reported.

Defense Ministry sources said that U.S. Ambassador Deane R. Hinton met with the Army high command, which went into continuous emergency session when the rebellion was announced.

UPI added that a State Department official in Washington said, "We don't regard this as an attempted coup. There have been no other preemptive acts. We are watching and waiting, watching this closely."

In a communique, Ochoa called for "moral support from our colleagues so we may reach our objective." Provisional President Alvaro Magana should take over command of the armed forces and exercise it "through the armed forces high command," the communique added.

As president, Magana in theory already holds ultimate authority over the Army. In fact, however, the former banker was picked as president in large measure by the Army, and Gen. Garcia acts as effective commander. Magana made no statement on Ochoa's demands.

Garcia met with his top officers throughout the day, an Army communique said. An Army spokesman, Col. Eusebio Cotto, said they reached no decision on whether to move against Ochoa. But Col. Adolfo Blandon, commander of the 1st Infantry Brigade in San Salvador, traveled to Sensuntepeque and delivered what he described as a message from the high command to Ochoa.

Blandon declined to comment on his mission. He stressed, however, that Ochoa's mutiny was a "family problem" that Blandon hoped would be resolved within the military. At midday, troops sealed off the main roads to Sensuntepeque, but there was no evidence of preparations for an attack.

Ochoa told reporters that Garcia sent him orders last night to quit his Cabanas command and leave for Montevideo, Uruguay, to become military attache at the Salvadoran Embassy there. This, he declared, would be tantamount to exile.

"I admire that country, but I'm not going," he told The Associated Press. "I would rather resign and grab hold of some other struggle front." It was unclear what other front he was referring to. Several officers have in the past switched sides to fight with the guerrillas.

Based on his past, however, Ochoa seemed more likely to be referring to political struggles. Ambitious and hard-working, the professional soldier of almost two decades often has been at odds with Garcia over conduct of the war in the three years that the minister has been in charge of the armed forces.

In 1972, according to acquaintances, Ochoa was part of a group that attempted a coup against then-president Fidel Sanchez Hernandez. After its failure, Ochoa was sent for 18 months as military attache in Costa Rica.

Ochoa also was reported to be among officers considering a move against Garcia three months ago. Their contacts were cut short, however, when Garcia called in the country's main officers and won a vote of confidence in an open confrontation, according to an informed military source.

Ochoa has received training from Taiwanese and Israeli officers. He attended military school here in the early 1960s with youths who went on to become key leaders in the clannish military establishment. Among them are the commanders of three rapid-deployment battalions trained by the United States. Also among them is D'Aubuisson, a political rival of Garcia who maintains independent ties to some officers in the Army, from which he was ousted.

Against this background, Garcia's orders naming Ochoa to Montevideo were seen as an attempt by the defense minister to rid himself of a troublesome subordinate whose link to D'Aubuisson made him potentially dangerous.

D'Aubuisson has criticized Garcia openly in the past few months for what he called failure to wage war against the guerrillas with sufficient vigor. He made no comment on the mutiny.

U.S. diplomats and military advisers here also have criticized Garcia's tactics in the war, which has taken approximately 35,000 lives. But they have backed the minister as the country's best military leader, partly on the consideration that he recognizes human rights sensitivities in the U.S. Congress and that a minister in D'Aubuisson's mold would risk leading the country into such repression that Congress would cut off aid.

The United States has provided military aid totaling $125 million over the last two years and is planning another $60 million in the coming budget.

To continue the aid, the Reagan administration must certify periodically to Congress that the government here is making progress in correcting human rights abuses, with the next certification due later this month.

Ochoa took over Cabanas Department, as provinces are called here, in August 1981. Within six months, he was credited with cleaning the area of guerrilla activity, mostly by repeated small-unit patrols. Although Ochoa was not U.S.-trained and is said to maintain his distance from the U.S. advisers here, his tactics exemplified those urged on other Salvadoran commanders by their U.S. trainers.

Ochoa's troops included civil defense units whose tactics were sometimes attacked as brutal but which provided him with close-to-the-ground intelligence on guerrillas and ready contact with the civilian population. Observers here say the success of his strategy, which amounted to virtual occupation of the area, has won him respect from many Cabanas residents.