After a decade of a gigantic building boom fueled by spectacular oil wealth, Saudi Arabia is entering an era of multibillion-dollar deficits and relative austerity whose full implications for the government and society are still far from clear.

Saudi officials say the kingdom, which spent $362 billion on projects between 1975 and 1982, has built virtually all it wanted or needed, and that it welcomes a respite in the headlong rush from the camel to the computer age.

"We are going through a new era of our society," said Abdul Rahman Zamil, deputy minister of commerce, in an interview. "It is an era that will be extremely positive to the Saudis, an era that will be defined by clarity of objectives, clarity of priorities and an extreme cut in the fat that we have been living with within our own projects and decisions."

No one can doubt that the Saudis had become used to a lot of "fat," what with oil production peaking at slightly more than 10 million barrels a day in early 1981, revenues at $112 billion that year and foreign reserves between $150 billion and $170 billion. This is with a population of probably 5 million.

By comparison, the highest revenue predicted for this fiscal year, which ends next April, is $59 billion.

The sudden drop in spending could prove jolting even to this well-cushioned society that has become accustomed to easy money, huge profits and little effort.

"It's a very rich man who is suddenly less rich, and it's very painful," remarked one western resident with access to the royal family. "They feel poor."

Among the most serious potential political effects for the ruling house of Saud, according to some western analysts here, is a possible breach in its strong alliance with the new merchant and entrepreneurial class that has grown enormously in size and wealth during the past decade, thanks to government outlays.

Already, the government has moved to protect this class by instituting a kind of "protectionism" against foreign competition, assuring Saudi bankers and businessmen a big share of the shrinking market. A recent royal decree stated that foreign contractors must give at least 30 percent of subcontracts to Saudi companies, while the all-powerful government monetary agency, Sama, has tightened restrictions on loans by Bahrain's offshore foreign banks to assure Saudi banks priority locally.

The big question is whether this emerging Saudi business class is ready to stand on its own feet, without huge government infusions. The Saudi economy, once dominated by the oil industry, has changed radically during the past decade as with the expansion of the private and non-oil sectors. Last year, the latter accounted for $60 billion in business, or almost 40 percent of the gross domestic product, according to the Saudi-American Bank.

Zamil admitted, however, that the private sector still depended on government infusions and that there was a risk that "some will be hurt" in the new era "of a real business professional society." He insisted that the benefits far outweighed the risks.

"Every businessmen now has entered into his mind the psychology that it is not easy to make a buck. You have to fight for it. You have to struggle and you have to be competitive. You can feel it everywhere," he said.

Saudi officials have reacted angrily to gloom-and-doom reports about the economy from abroad and sought to assure local and foreign investors that all is still going well, despite accumulating evidence to the contrary. In early April, the influential finance and national economy minister, Mohammed Abalkhail, denied that any big projects were being canceled and said the kingdom was still forecasting an 8 percent annual growth rate "for many years to come."

The government is continuing to pump money into the less favored provinces and basic social services to prevent public discontent, but it has budgeted nearly a 20 percent cut in the $3.2 billion spent last year on subsidies to keep key food items, gasoline and electricity at absurdly low prices.

There is also a $2 billion cutback from last year's $5 billion for starting new projects. That was already a big drop from previous years. An economic slowdown thus seems inevitable.

Opinions vary on how serious a jolt the Saudi economy can expect. A $235 billion five-year development plan, with two years to run, has, in the words of one western economist, seen "the bottom fall out of it."

But the plan was never more than a loose collection of "wish projects" by various ministries. "They probably had already overbuilt anyway," remarked the economist.

Most economists and bankers interviewed here seem to view the Saudi economy as in "readjustment" to a far lower level of spending and in transition from construction to maintenance and services.

The first visible adverse effects of this "readjustment" have included layoffs of mostly foreign workers at such poles of activity as the Jubail and Yanbu industrial centers as well as at the giant Arabian-American Oil Co. (Aramco), the stretching out of multiyear projects, open bidding on contracts to reduce prices and once lucrative commissions to agents, and long delays in payment of bills--a practice that is driving some companies to the edge of bankruptcy.

The delays, which Saudi officials describe as "technical" rather than "fiscal policy," are averaging 90 to 120 days, but in some cases have gone on for six months. Analysts of the Saudi budget believe this is one way the government achieved a 22 percent cutback in the budget last year.

Some western economists say the government has gone into a holding pattern on spending and payments until it becomes clearer what the new plateau of its oil revenues will be later this year.

Finance Minister Abalkhail said recently that the government was waiting at least until August before taking any major fiscal decisions, such as whether to dip into foreign reserves or cut back even further on this year's already sharply reduced $75.6 billion budget. But outsiders predict that only after the annual pilgrimage, or hajj, to Mecca in September will hard decisions be taken and "blood will be spilled," as one economist put it.

Everything seems to hang on the big unknown of Saudi oil production and this, in turn, depends on the state of the world economy. Guessing Saudi oil exports, and thus the size of Saudi income and deficits, has become the biggest game in town.

It makes a big difference not only to the Saudi government but also to foreign banks holding billions of dollars in the kingdom's assets and foreign contractors with projects under way here.

So far this year, Saudi production has varied from below 3 million barrels a day to slightly more than 4 million--still far below the 5.5 million barrels that analysts here surmise was used as a basis for the 1983-84 budget.

If the kingdom only averages 4.5 million barrels a day this fiscal year, which seems a possibility, then its oil exports would bring in only about $38 billion. Saudi Arabia now consumes 800,000 barrels daily of its own production, which would leave only 3.7 million for exports.

Saudi oil income would thus be only about half of the last calendar year's estimated earnings of $77 billion and slightly more than one-third of those in 1981, the peak year. But the kingdom also expects to earn more than $13 billion on its overseas investments this year, plus another $7.7 billion from gas sales and other items.

Whatever its exact oil earnings, the kingdom seems headed for a huge deficit for the first time in its contemporary history. The government has predicted a $10 billion deficit, breaking with a 20-year tradition of carefully balanced budget plans despite several unexpected small deficits.

This is as shocking to Saudis and foreign bankers and businessmen as a $100 billion deficit in the U.S. budget used to be to Americans. But the Saudi forecast is, according to analysts of the Saudi budget abroad, extremely optimistic.

Vahan Zanoyan, director of the Washington-based Wharton Middle East Econometric Service, has predicted a deficit of about $21 billion and Chase Mahattan's Middle East specialist, Sharif Ghalib, only slightly less. Other estimates go as high as $26 billion if oil production remains only at 4 million barrels a day.

While Saudi Arabia has no foreign debt and easily could arrange loans to fill the gap, it seems certain first to tap its huge foreign reserves. Some bankers and economists here believe it has already dipped into them for $5 billion to $6 billion to meet the last fiscal year's actual expenditures of $70 billion.

Outside analysts seem to concur in the view that Saudi Arabia can tap its reserves to cover budget deficits in the next two years without major problems. But if its oil production continues at a low level for a longer period, they say, the kingdom could face a far more radical cutback in spending and the Saudi style of living than just the simple "readjustment" apparently now in the works.