Faced with a dramatic decline in its oil revenues, Nigeria has slapped a freeze on virtually all imports, a decision that could jeopardize billions of dollars worth of development programs and increase social unrest in Africa's most populous nation.

The News Agency of Nigeria said today that the country's central bank ordered commercial banks on Tuesday to stop issuing letters of credit, the means by which most imports are financed, Reuter reported from Lagos. Nigeria's imports totaled $1.8 billion last month.

The move follows months of widely fluctuating oil sales and steadily falling revenue for black Africa's premier oil exporter and a major supplier to the United States after Saudi Arabia. In the past two years, Nigeria's petroleum earnings, which account for 85 percent of government revenues, have dropped by more than half.

The instructions to the banks followed last weekend's emergency session of the Organization of Petroleum Exporting Countries in Vienna at which Nigeria agreed to further cutbacks in both sales and prices for its low-sulfur crude. The reductions are part of an OPEC-wide 700,000-barrel-a-day cutback hammered out in an effort to stem the recent oil glut that has undercut the cartel's prices and played havoc with its members' revenues.

The Nigerian import freeze comes on top of deep and unpopular budget cuts also resulting from declining oil revenues. Like in other Third World petroleum exporters, Nigeria's leaders politicians have promised--and its 100 million people have come to expect--imminent affluence brought about by the country's billions of oil dollars.

The flegling civilian government of President Shehu Shagari, who took office in 1979 after 13 years of military rule, is under intense pressure to take the shortest, most costly road to high industrial wages, modernized farming, free public education, extensive medical care and social security programs. The government faces the delicate task of containing the increasingly militant salary demands of its labor unions and the mounting economic aspirations of the country's subsistence-farming peasants who constitute 80 percent of the population.

The dramatic import freeze reflects a deepening financial crisis that the government has been struggling to avoid since last July. Since then, Nigeria's foreign exchange reserves have plummeted from a $8 billion to $2.7 billion, according to Amusa Otiti, the bank's director of foreign operations, quoted in the Nigerian news agency report.

Nigeria's current austerity budget of $20 billion--down from last year's $23 billion--was to be financed by projected oil exports of 1.3 million barrels a day at $36.50 per barrel. But with last weekend's OPEC agreement, Nigeria's price will drop to $35.50 a barrel, in line with similar low-sulfur crudes produced by Libya and Algeria, and further reduce expected earnings.

Oil industry sources said Saudi Arabia has promised Nigeria and other vulnerable OPEC members that it would bear the brunt of any future production cuts. There are also unconfirmed reports that the Saudis, along with Kuwait and the United Arab Emirates, have promised to supply as much as $1 billion in budget support loans to Nigeria if the government is forced to cut production below the 1.3 million barrel mark.

Although Saudi Oil Minister Ahmed Zaki Yamani denied it publicly last weekend, industry sources said the loan offer was first made at the meeting of key OPEC ministers earlier this month in Doha, Qatar.

Since January, Nigeria's production already had fallen from 1.7 million barrels a day to 1.4 million as regular customers went elsewhere for cheaper, similarly "sweet" crudes such as British North Sea oil.

With one disastrous exception, Nigeria has adhered closely to OPEC's pricing structure and stayed in close alignment with Libya and Algeria. The exception was in 1977 when a hasty increase in oil prices by the former military government caused demand for Nigerian oil to drop sharply and left the country with a $4 billion deficit.

It was unclear today how long the freeze would last and what impact it would have on Nigeria's ambitious $125 billion development plan. In its instructions Tuesday, the central bank ordered the commercial banks not to extend or renew existing imports commitments or apply for new letters of credit until further notice.

One member of Shagari's staff said recently that that the government still plans to carry out most of the development plan, including $13 billion for agricultural projects. But last month Nigeria scrapped a $15 billion plan to build a liquefied natural gas facility.

About 50 percent of Nigeria's import budget is spent on items such as heavy machinery, power production equipment and spare parts, and the rest is split between raw materials and consumer goods. Nigeria also imports about $1.5 billion a year in food.

Many of Nigeria's 19 states have run out of their budgeted share of oil revenue. In one state, Kaduna, teachers have refused to work for the past eight weeks because the government has not paid their salaries for the past three months, Nigerian sources said recently.

Workers for the Nigerian Electric Power Authority went on a six-day strike in early March cutting off all electrical power and virtually shutting down the entire country. The strike precipitated a health crisis as well when urban water supplies, reliant on electric pumps, dried up. The government, after threatening to fire the electrical workers, backed down and hastily agreed to pay the much delayed bonus.

Similar flareups, both in public employment and private industry, have begun to spread recently as the economy slows into a recession.

For a brief period last August, after the country's oil sales had dropped to an average of 700,000 barrels a day, Nigeria appeared to bow to market pressures. as it did in 1977, It broke away from OPEC pricing agreements by dropping its price from $40 a barrel to $34.50. But when production increased again last November, Nigeria raised its prices by $2 a barrel and returned to conformance with the cartel.

Despite its present willingness to adhere to OPEC's price structure, pressures on Nigeria remain intense. The non-OPEC British National Oil Corp., which used to tie its North Sea oil prices to Nigeria's, has dropped its price for low-sulfur crude to $31 a barrel--$4.50 less than Nigeria's.