WHILE NEWSPAPER headlines scream imminent catastrophy, economists and bankers predict financial collapse and guerrillas and government troops arm for a resumption of civil war, officials of Nicaraguan President Anastasio Somoza's Liberal Party are preoccupied with a more traditional concern.

Tuesday is the president's birthday. As in the past, it is planned to be a day of celebration in government and party circles, with local officials expecting small gifts from the population. Normally, celebrations in Managua call for an evening serenade by party stalwarts, the gathering of local street children in a Roman Catholic church for services sponsored by the president's mistress and a National Guard ceremony in honor of Commander-in-Chief Somoza.

The most complex preparations, however, involve the selection and purchase, with "voluntary" party and business contributions, of the president's birthday present. Considered Nicaragua's best kept yearly secret, the present is chosen by a party committee that struggles to come up with a gift suitable for a man who appears to have everything.

In past years, the party has presented Somoza with several dozen prize breeding pigs for his farms, a yacht and a roomful of exercise equipment.

"What we'd like to give him this year," one opposition business leader said," is a one-way ticket to Uganda."

The birthday preparations lend an additional element of surrealism to what is already a dreamlike atmosphere here.

While life on the surface is relatively normal, Nicaragua is on the edge of its nerves, gripped by collective anxiety that has reduced daily social interaction to a series of questions: What you have heard? When will it be?

The what is the next large-scale offensive by Sandinista National Liberation Front guerrillas who led a three-week civil war last September. The when is anybody's guess.

There are repeated false alarms - a border clash when a group of Sandinistas cross into Nicaragua from Honduras to the north or Costa Rica to the south and encounter a National Guard patrol, an alleged Sandinista communique giving a 36-hour attack deadline that passes without event.

Meanwhile, a well-to-do Managua woman, the wife of a businessman, repeatedly sifts through her closet, weeding out all but essentials, giving the castoffs to the poor and moving the remainder to a small room in the back of the house.

The room is carefully chosen for the strength of its walls, its concrete ceiling and its distance from the open courtyard. It is here that she and her husband and servants will go when the attack comes. Several weeks ago, she sent her children to live with relatives outside the country.

In a slum settlement several miles away, youths meet nightly to turn tin cans into homemade bombs, hold political classes and discuss how to deal with "orejas," the government's "ears." Neighbors suspected of being government spies are dealt with harshly. Their homes are sometimes burned. If they are lucky, they escape alive.

Parents gather nonperishable food, medicine and bandages in caches and keep their own ears tuned for the distant rumble of explosions and the occasional pop of rifle fire. Assured that the gunfire is merely an isolated rebel clash with the national guard and not the beginning of the offensive, they resume quiet talks over when and where it will come.

AFTER SEVEN WEEKS of U.S.-sponsored mediation between it and Somoza, the opposition coalition of political, labor and business leaders demanding the president's resignation has been thrown into disarray.

Despite the mediators' sidetrips around composition of a provisional government to replace Somoza, the niceties of supervising a nationwide referendum and the question of how to judge corrupt National Guard generals, the opposition has known all along there is only one issue: will Somoza stay or go?

After more than 40 years of Somoza family rule, Nicaraguans believe they know their president well. He will not go peacefully.

Now that the mediation appears to have failed, and the frenzy of meetings and negotiations has slowed to nervous waiting, the opposition tends to blame the United States for cajoling it into small concessions based on a U.S. commitment to push Somoza from office.

While the Sandinistas said all along that the effort would fail, the political position's willingness to believe in the United States, or at least to believe that their huge northern neighbor had the final say in Nicaraguan events, was as much a part of their traditions as the rule of the Somoza family.

"We've been at this for weeks waiting for Uncle Sam to get him out of here," one opposition leader said, "and we finally realize tht it's up to us."

"But that realization has not resulted in any cohesive opposition plan or any lessening of the feeling that the Sandinistas and Somoza's National Guard are once again on a collision course whose crash they are powerless to prevent.

There are many excuses why the Board Opposition Front has been unable to hold itself together and lead the course of events here.

As in many Latin American countries where political activity has been severely restricted and a powerful dictator or military regime has made itself invulnerable, many sectors of Nicaraguan opposition have nver had or taken the chance to adequately organize themselves and build power bases. Recognized leaders have not risen to the surface, because the surface has long been occupied with the Somoza family, the Liberal Party and the military.

With little to tie them together except their common desire for Somoza's departure and with renewed violence apparently inevitable, the opposition factions have begun tosplit.

The traditional business community, which sees the final blow to its productive capability on the way, is more willing than many of the others to keep plugging away at the mediator's proposed referendum on Somoza's continuing presidency, despite the fact that Somoza turned it down flat.

While they recognize that the guerrillas must play a part in any solution to the Nicaraguan crisis, the conservative businessmen's nervousness over the Sandinistas is almost as great as its fear of the National Guard.

While they have tactitly supported the guerrillas, politicians of the traditional opposition Conservative Party fear that a Sandinistas victory will lessen their chances for articipation in any future government, or at best will give them less than the little influence they have now.

Labor groups feel they are being sold out by the businessmen, and the Young Turks of opposition politics view themselves as the only ones with guts enough to stand up to both Somoza and the United States whatever the consequences.

The waiting is getting to everyone.

The United States appears as much at a loss for new ideas as is the opposition. Its mediators are still pushing the referendum idea and looking for concessions from one side or the other.

While it intended to partially support the referendum, with certain conditions, and pulled back to reject it only after it heard Somoza had turned the idea down last Friday. The opposition was and still is beset with a pervasive feart hat Somoza could win such a vote.

Although to the American mind that fear may translate into a sneaking suspicion that perhaps the anti-Somoza feeling in Nicaragua is not as strong as the opposition has said, the opposition maintains that that interpretation denies recent Nicaraguan history.

In the first place, despite their bravado, the opposition sectors are well aware that because of Somozas political domination for so many years, few Nicaraguans outside the educated urban classes know much about who they are and what they stand for.

On the other hand, they believe the sandinistas have captured the hearts and minds of the politically uneducated Voter turnout for a referendum between them and Somoza, some sector of the opposition feel, would be scant unless they had the direct endorsement of the guerrillas.

Longtime intimidation has boosted the popular belief in Somoza's ability to control events here to the level of legend. Despite all guaranteed international controls proposed by the mediators, there is a pervasive feeling that, even if he did not win fairly, Somoza would end up with a referendum victory.

Finally, even if Somoza did promise to step down should he lose the U.S. proposed referendum, there is little faith that he would keep his word.

As the mediation falls apart and the opposition feels its hands tied, only two actors in the Nicaraguan drama seem unperturbed and sure of their direction - Somoza and the Sandinistas.