The Sudanese Army's announcement today that it was seizing power for "an interim period" followed the now-classic African coup pattern, including the obligatory promise of a return to civilian rule.

Whatever happens -- and contemporary Third World history is strewn with examples of armed forces remaining in power after similar pledges -- the immediate stimulus for ending President Jaafar Nimeri's 16-year reign came from Sudan's civilian elite and not from the armed forces.

Nor does the Reagan administration emerge with its credibility undamaged, for, despite numerous storm warnings, it publicly and repeatedly embraced Nimeri long after it had become obvious that Washington alone was supporting his increasingly unpopular rule.

Nimeri's downfall came about with the realization by the doctors, lawyers, engineers and other middle-class professionals that the president was politically isolated at last and could no longer rule by dividing the opposition.

The last card in his well-used deck was the arrest last month of the leaders of the Moslem Brotherhood, the Islamic fundamentalist sect that had persuaded him to impose sharia, or koranic, law on an actively hostile animist and Christian south and a scarcely more enthusiastic Moslem north.

Faced with rebellion in the south, widespread drought, a massive influx of refugees from Ethiopia and the consequences of his mismanaged economy, Nimeri helped bring about his own downfall by pushing his contempt for his adversaries too far. He all but dared them to get together while he flew off for a visit to the United States even as rioters were ransacking his capital.

After those violent initial demonstrations, it took the professional elite a week to organize their own peaceful processions, which at times seemed a throwback to the nationalist uprisings of 19th century Europe.

Their goal was to provoke a rerun of the "October revolution" of 1964. At that time, intellectuals engineered the downfall of the military government headed by field marshal Ibrahim Abboud.

A Khartoum University professor acknowledged last week that this goal was probably unattainable and predicted, "I know Nimeri will probably be replaced by another military man, but anyone would be better, even Idi Amin," the ousted Ugandan dictator.

Within 24 hours of the first demonstration by mostly middle-class and middle-aged Sudanese in Khartoum Wednesday, the leader of the two-year-old insurrection in the south announced that he was negotiating with the Sudanese armed forces. That rebellion, led by John Garang, a southern Christian and an American-educated former Army officer, had demanded Nimeri's overthrow from the start.

This confirmed the professionsal elite's hunch that the armed forces initially would refuse to take part in crowd control operations, and then could be won over to deposing Nimeri.

Just how far the Army leaders are willing to go remains open to question, since they were handpicked Nimeri supporters only yesterday.

However the Sudanese play out their political future, at first glance Nimeri's downfall is a blow to the Reagan administration.

From the president on down, U.S. officials praised the Sudanese leader in language that now seems as ironic as president Carter's ill-timed endorsement of shah Mohammad Reza Pahlavi on the eve of the Iranian revolution that was to sweep him from power.

But if U.S. concern for both Iran then and Sudan now turned on strategic considerations, the Reagan administration stands a better chance than Carter did in Iran.

Unlike Iran, Sudan is not an oil-rich power. Rather, it is one of the poorest and most ill-managed of Third World countries. It also is dependent on U.S. grain shipments to avoid mass starvation among its population of 22 million.

That leverage alone -- if tactfully manipulated -- should be more than enough to prevent any radical antiwestern government from taking over in Khartoum, according to observers.

Neither the Army nor most of the professional elite is antiwestern, although many intellectuals are furious that the United States insisted on backing Nimeri to the bitter end.

The Sudanese elite, one of the most respected in the Third World, has convinced itself that the Reagan administration did so largely because it did not have the courage to live with the uncertainties that necessarily must follow Nimeri's removal.

Yet Sudanese intellectuals argued that the United States, Egypt and Saudi Arabia, Sudan's most concerned allies, should be relieved to be rid of Nimeri and his needless complications of the country's massive problems.

U.S. interests in Sudan traditionally are described as reflecting the concerns of its Arab allies, Egypt and Saudi Arabia.

Egypt historically has worried lest the Nile waters flowing through Sudan on the way north to the Mediterranean fall into hostile hands.

Saudi Arabia has a similar interest in keeping the western shore of the Red Sea under the control of a friendly government.

With Nimeri's disgrace, Garang's Sudanese People's Liberation Movement may be able to come to terms with any new government in Khartoum and thus shed its tactical dependence on Libya and Ethiopia, both of which aided his rebellion to settle old scores with Nimeri. That should allay Washington's fears that Libya's Muammar Qaddafi, who rushed to recognize the new Khartoum government, actually stands much chance of influencing events there.