With opposition to military rule growing throughout Chile, Gen. Augusto Pinochet's hold on power has been largely sustained by a meticulously constructed base of support within the Chilean armed forces.

South America's worst economic recession and three months of mounting public pressure have stripped the Chilean president of much of what was once a substantial reserve of popular backing and plunged his decade-old government into an atmosphere of political crisis.

Despite challenges that could spell the downfall of many Latin American governments, however, opposition leaders say Pinochet's position may be far stronger than it appears to be.

Public disaffection, these leaders say, has yet to produce any concrete evidence of rebellion in Chile's military. The armed forces, particularly the Army, are seen as the only possible agents of change in government or in Pinochet's plans to rule until at least 1989.

As resistance to Pinochet has mounted this year, there have been signs of discontent within the Chilean Navy and Air Force, according to informed political and diplomatic sources. But Pinochet, the Army commander in chief, has fashioned a formidable structure of control in his service that would block all but an overwhelming move against him, these sources say.

In preserving this structure, Pinochet has been helped by a long tradition of professionalism and respect for hierarchy that distinguishes Chile's armed forces from those of other Latin American nations. So separated from civilian society are most military officers that even politicians supporting the government say they have little contact with the services and almost no knowledge of political developments within them.

As a result, no one knows what role Chile's military would play in a confrontation between Pinochet and his opposition. And that role is pivotal.

"The opposition could win over the better part of the country," one party leader said. "But if Pinochet keeps the Army, he will never be threatened."

Civilians' general lack of knowledge about the military's political atmosphere has helped provoke a spate of rumors in recent weeks, and the speculation has been heightened by the appearance of mysterious documents.

In May, a letter was sent to the homes of several high-ranking generals purporting to represent the views of an unnamed group of officers and describing the need for "a change in the helm."

"It is preferable to alter our sacred institutionality one more time before living a future of ruin and dishonor," said the document, which has circulated widely in political and diplomatic circles. According to informed sources, several generals forwarded copies of the letter to Pinochet.

More recently, a second, 24-page document has been sent to military officials outlining alleged corruption involving a member of Pinochet's family.

Accompanied by copies of official documents and contracts as evidence, the paper alleges that Pinochet's son-in-law, Julio Ponce, has amassed a fortune in cattle, fishing and lumber operations while serving in a series of top government posts.

Ponce, the report charges, has operated a million acres of wooded government land in southern Chile "like his own estate" and has accumulated other large properties in Chile, Paraguay, Argentina and California.

This "true national scandal," the document says, "has been denounced to the president and the junta of generals."

Signed only, "Committee for Dignity of Chile," the newer document deals with a topic that pro-government conservatives believe is of deep concern to the military.

In recent years several members of Pinochet's family, including Ponce and a daughter, Lucia, apparently have been enriched by government funds, according to several government supporters. Pinochet recently felt obliged to defend himself to his generals for building a palatial home outside of Santiago at a cost estimated at $9 million or more, the sources said.

No one thinks either document originated within the ranks of active-duty officers, but as one observer put it, "The question is not so much who wrote them but what the reaction to them was."

There is general agreement that the reaction cannot be measured reliably. One former government official and another source with military contacts said that Army leaders were believed to have asked for an internal investigation of corruption charges. Those reports were strengthened this week when Ponce suddenly resigned his latest government post as general manager of a state development agency.

These sources and other observers added that many officers in the Chilean Navy and Air Force were believed to be dissatisfied with Pinochet. However, they said that these services could not act without the full support of the Army, and in that branch, Pinochet's support was believed to be strong.

These conclusions are based in part on the Army's long history of strict respect for hierarchy and isolation from politics. Until the coup that ousted the Socialist government of Salvador Allende in 1973, Chile's military had been uninvolved in politics for more than 40 years. So scant was civilian contact with officers that many of Allende's top supporters believed Pinochet, then the Army commander, was committed to defending the democratically elected government against a coup.

In the past 10 years, Pinochet has accentuated the traditional barriers to military political action. Through major reforms and systems of promotion, retirement and internal intelligence, he has concentrated power at the top of the command and eliminated any institutional means of questioning his authority.

Instead of making retirement mandatory with age or length of service, Chilean law now specifies that the service heads can be removed only in the case of "death, resignation, or absolute impossibility" of continuing to serve. In the case of Pinochet, "absolute impossibility" can be determined only by the junta, and the Army member leads the junta.

Within the Army, the structure of leadership has changed markedly in 10 years. Pinochet has doubled the number of generals, from 25 to 50, thereby diluting authority and satisfying many officers' ambitions. The most senior generals with Army commands are at least nine service classes behind the commander in chief in seniority. Many attended the military academy when Pinochet was an instructor there.

In separating the bulk of the Army from the government, Pinochet has relied on military government ministers who have passed the normal retirement age and remain on active duty by special dispensation of the commander in chief. This means that Pinochet can order the retirement of generals serving in the government at any time.

Military authorities say two bold moves by Pinochet have given him potent weapons against rebellion. One was the consolidation of the services' separate intelligence agencies into a single command directly answerable to the president, a move that has given Pinochet a monopoly on information.

The other was his ouster of Gustavo Leigh as commander in chief of the Air Force in 1978 through a decree of the Interior Ministry, demonstrating that he holds ultimate power over the chiefs of the other armed services.

Since Leigh's forced departure along with about 20 other Air Force commanders, no military leader has been known to challenge Pinochet openly.

"He has eliminated all potential opposition," said one Chilean military scholar. "The people at the top are there because they show no potential of being a threat."

The relative weakness of leadership in the top ranks and Pinochet's long monopoly on authority also have led some analysts to believe that the options of even rebellious officers have been closed off.

"If there were a real alternative to the present government there's no doubt there would be a lot of pressure to move toward it," said one ranking conservative with personal ties to Pinochet and other officers. "But it would have to be very clear that a replacement would have the power and authority to govern. There's just no alternative like that."

Other military analysts argue that officers compromised by years of commitment to the current system and deeply antagonistic to Chile's traditional democratic political parties have not been swayed so quickly as civilians by the government's policy failures.

"There is simply no way that the armed forces would give up the constitution they are committed to and turn over the government to the same politicians who had it before," said Mario Arnello, a conservative former congressman.

For the armed forces to move against Pinochet, many analysts conclude, officers would have to be convinced that they were incapable of controlling rising social unrest and violence and that Chilean opposition leaders offered a safe alternative that preserved military prestige.

"If the Army is called out to maintain order, they will do that," a former government official said. "If the protests go on and they find themselves shooting people to stop them, they could rebel. But it would have to be a very extreme situation."

While most analysts say that such a move against Pinochet would still take months to develop, all agree that the actions of Chile's military cannot be predicted reliably.

"We all talk, but no one really knows," said an opposition organizer. "It is a factor that will never be certain until we wake up some morning and find tanks in the street."