To most Zairians and many whites here, the biggest threat to everyday existence comes not from the rebels, but from Zaire's own army.

Born in mutiny in the first days of independence from Belgium in 1960, the 45,000-strong force has become synonymous with extortion, looting murder, rape - not with protecting the country or citizenry.

Diplomats fear the success of current Western efforts to put Zaire back on sound economic and administrative rails may be undercut by this military force that acts like an army of occupation in its own country.

Trained on-and-off since 1960 by Belgians, French, Italians, Israelis, North Koreans, Chinese and Americans, the army, according to Western military specialists, is without effective noncommissioned officers, reserves or a meaningful general staff.

In normal times the only regularly fed and paid unit is the presidential guard, chosen from President Mobutu Seae Seko's northwestern home province of Equateur. The guard's long suit is loyalty: Western military specialists are not impressed by its professional training nor by its discipline.

"If the president thinks he is adequately protected by the guard," a military attache said, "he is woefully mistaken."

According to Zairian citizens, when the guard launched a punitive expedition in the interior last winter as many as 800 unarmed villagers were killed.

The officer corps has been noted for absenteeism, with many officers staying at home or looking after business interests.

Theoretically, privates are paid $60 a month at the official exchange rate - a sum which they and other Zairians find difficult to live on. But since the troops are paid only intermittently, they have taken to setting up road-blocks to extort money from passersby, ordinary Zairians complain.

Recruiting has often consisted of rounding up young men at random on city streets, cordoning off neighborhoods or emptying prisons.

Even the crack Kamenyola Division, trained by North Koreans, lost two battalions last month when officers and men melted into the bush near the road-and-rail hub of Mutshatsha during the rebel invasion of Shaba. They have yet to reemarge, informed sources reported.

Mobutu appears to distrust the military to the point that some Western military specialists question whether the one-time sergeant really wants an efficient army, since it could threaten him. Within the past year, he removed all army officers from the important Politburo of the country's one party, the Popular Revolutionary Movement.

Following a spring trial of 84 military defendants accused of plotting, 13 of those convicted were executed before their lawyers could file appeals.

Little meaningful evidence was produced at the trial, according to independent observers authorized to follow the proceedings.

Just before the renewed fighting in Shaba last month, some 200 officers and warrant officers wre cashiered, Western sources said, because they belong to the Baluda and Lunda tribes that are important in volatile Kasai and Shaba provinces, respectively.

After that move, according to Western specialists, the general staff - which had shown marked improvement after the last Shaba invasion of last year - ceased to be effective.

The present decline of the army dates from its brief and disastrous engagement during the Angolan civil war in 1975 when, with U.S. backing, its crack units were committed to help stop the eventual winners - the Popular Movement for the Liberation of Angola.

They abandoned considerable equipment, much of it American, and returned with their morale shattered. Even the wounded were hidden since Zaire did not want to publicize its participation in the Angola fighting.

In last year's Shaba campaign, the Westerners say, the high command committed troops to the combat zone without sufficient food, ammunition, or cadres.

Apparently mindful of the mess in the military, Mobutu recently said Belgium would train a 10,000-man infantry division, France would accelerate formation of 30,000 paratroopers and Morocco would also help train new troops.

But privately, Belgian and French officials have cast doubts on such reports. Belgian officers insisted that in the past their training missions did not have sufficient authority to exercise discipline.

Some diplomats still hope that a lean force of 15,000 to 20,000 men could be formed, with most of the present troops cashiered.

That was the approximate size of the old Belgian colonial force which maintained law and order in pre-independent days throughout this land as large as the United States east of the Mississippi.

Some Western specialists contend that the only way to achieve an effective and disciplined army is by having white officers perhaps as far down as battalion or even company level.

"That is politically out these days," a specialist said, "if only becausethe Russians and Cubans would seize on it as a pretext to justify their own military operations in Africa and even expand them."

Nor are mercenaries, a traditional recourse in Zaire's early independent history, considered politically palatable in the West in these days of open government.

"The army is likely to bring th eregime down through its own mindlessness," a Western diplomat said. "It all sounds like a rerun of Chiang Kai-shek's army in the Chinese civil war."

For the time being, an African expedionary force is propping up the regime. According to a Western specialist, though, "the only way out for Mobutu is a political solution. Failing that, the Western powers are going to have to face the possibility of more rescue operations along the lines of the Shaba interventions of the past two years."