As the sun rises over Managua, the regular night-time sound of bomb explosions and rifle fire gradually gives way to the normal daylight racket of vehicles and voices.

But on a hill above the growing urban din, the only sound to be heard is the soft thump of padding feet. It is 6:45 a.m. and President Anastasio Somoza, overseen by armed soldiers posted in guard towers, is beginning his morning calisthenics with a jog around his house.

It is a happy morning for Somoza. The day before, in a large antigovernment demonstration, only seven people were reported wounded in a postprotest shootout with National Guard troops. In view of the violence that was expected, Somoza says in a jogalong intervies, "it can be considered a day without incidents."

Thirty-four laps, ten situps, and five miles on a stationary exercise bicycle later, he notes that since September's guerrilla-led civil war, Nicaragua "has moved from a complete state of violence to a complete state of tranquility."

In his own terms, perhaps, Somoza's assessment of the situation is valid. His personal fortune and power, despite U.S. demands that he resign, are relatively intact.

As long as he maintains the loyalty of the military. the wealth and position of whose leaders are intimately tied to his own, the air on Somoza's well guarded hilltop is sweet and calm.

But if Nicaragua is peaceful today, it is only in comparison to what has gone before and what a great many Nicaraguans believe is sure to come again.

The military presence in Nicaragua's cities and their slums appears at its highest since September. Scores of youths, described as suspected terrorists, are picked up weekly and jailed either openly or without government acknowledgments. Sometimes they are released; occasionally they are later found dead.

Street gunfights and homemade bomb explosions are so common that most people don't even pause in conversation as long as the noise seems far enough away.

Guerrilla and National Guard activity is heavy near both the northern and southern borders, with automatic weapons, mortars and air-launched rockets reportedly in use. Guerrilla and government claims of enemy dead totaled in the dozens last week.

Concurrent with the crescendo of terror and repression is the near total disintegration of the anti-Somoza political, labor and business coalition that has been negotiating with the government through a U.S.-led international mediation team.

From its original 15 members, the Broad Opposition Front (FAO by its Spanish acronym) is down to 10 groups and, according to one informed analyst, those remaining have become "more rightist and more reactionary."

Those who quit FAO have joined leftist students, peasant and worker groupings -- including some allied to the Sandinista Liberation Front guerrillas and the Nicaraguan Communist Party -- to form the Patriotic Front.

The basis for division between the two fronts is who will or will not allow the United States to participate in finding a solution to the Nicaraguan crisis. In the eyes of many, the U.S. government has discredited itself, and the FAO along with it, by repeatedly backing down the the face of what they see as Somoza's intransigence.

Last Friday, after a two-week absence following Somoza's rejection of what they had called a final mediation proposal, the mediators returned here and announced that they had modified the proposal to conform with Somoza's demands.

The U.S. position throughout the three months of mediation has been based on several premises. The first is that lasting peace in Nicaragua is unlikely as long as Somoza remains in power.

This certainly has been colored, however, by strong concern is some sectors of the U.S. government that a violent overthrow would lead to control by Sandinista guerrillas whom they fear would install a government and endanger U.S. interests in the area.

Thus, the U.S. goal has been to get Somoza out without leaving a power vacuum into which the Sandinistas could jump. Joining in that goal have been the moderate FAO groups and leading businessmen whose fear of both continued war and an eventual guerrilla-led government outweighed their admiration for and support of Sandinista attacks against Somoza.

In early October, according to an informed diplomatic source here, the mediators "were initially incapable of articulating a way for Somoza to leave while saving face for him. Their first attitude was to ask him 'why don't you just resign?'"

U.S. mediator William G. Bowdler, Somoza said during a post-exercise breakfast, put the issue to him in their first meeting. "He said his instructions were to advise me to leave, to get out of the country."

On that occasion, and on many occasions since, Somoza said, his answer has been the same: No.

In one of the proposals and counter-proposals the mediators transmitted to each side, Somoza suggested a nationwide referendum on support for his government. If he lost, Somoza said, he would organize a new Cabinet, giving the opposition proportionate power under his continuing presidency.

To the FAO, such a proposal was anathema. During 45 years under the Somoza dynasty, repeated opposition "pacts" with the government ended in a sense of betrayal and the discrediting of any participating opposition faction.

The mediators, with FAO agreement, eventually adopted the plebiscite idea, but made it a referendum on whether Somoza should stay or immediately resign the presidency.

The proposal, presented in its ostensible final form Dec. 26, called for a plebiscite to be run and controlled by the Oraganization of American States the only way, a senior U.S. official said in Washington last month, to ensure a "fair and true test" in a country where government's electoral fraud is the rule.

Sources here said, and Somoza acknowledged, that the United States told him his rejection of the proposal would mean a reassessment of U.S. relations with his government. Within a week to 10 days, the four-man U.S. military mission here would be withdrawn.

That action would be followed by withdrawal of economic and finally diplomatic representation, one diplomat said, "of those programs most indicative of U.S. support" for Somoza.

Events of the past two weeks have outraged what remains of the FAO.Somoza immediately rejected the plan as a violation of Nicaraguan sovereighty and proposed a governmentrun plebiscite on the presidency. The mediators agreed to study the proposal.

When the mediators returned Friday, they had altered their "final" proposal into one allowing significant government control, with OAS "supervision" of a plebiscite.

Somoza has been given a week to reply.

Should he refuse, an informed source said, the United States has several options. The most extreme is U.S. military intervention which, he said, no one really takes seriously because of the political damage it would do to the Carter administration.

A second option might be OAS intervention with an international military force. So far, the United States cannot be assured of marshalling enough votes for such action within the OAS, dominated by rightist military governments.

There remains the plan for a gradual break in U.S.-Nicaraguan relations. But Somoza said the United States has already cut off economic and military aid funds and has "harassed" his government for two years.

Whatever influence the United States had in Nicaragua, Somoza said, has been lost.

"If they remove the military people and lower representation," he said, "materially I would not lose anything else but a few nice American gentlemen who are living in Nicaragua."

Outright U.S. opposition to Somoza or aid to his opponents would almost certainly fuel the fires of Somoza's friends in the U.S. Congress.Led by Rep. Charles Wilson (D-Tex.), they have already promised to sabotage the soon-to-be-debated legislation to implement the Panama Canal treaties.

Should Somoza accept the modified plebiscite plan, which seems likely, the question arises as to whose name would be on the other side of the ballot.

The delays and concessions have all but destroyed the FAO in both credibility and popular support and, as one of its leaders said last week, "hate amounting almost to a class struggle has grown" among the various opposition factions.

Not surprisingly, the break has come along the same ideological and economic lines that divided anti-Somoza factions last summer before they joined together and agreed, either actively or tacitly, to give the mediation a try.

While the Patriotic Front is still in its formative stages, its growing membership has refused all negotiation with Somoza or through any other country. Its organizers say plans instead call for "civic" opposition, including demonstrations and general strikes.

If the Patriotic Front succeeds, it will mark the first time in recent Nicaraguan history that grass-roots, popular forces have managed to organize and affect the course of events traditionally controlled by upper-class politicians.

In the meantime, despite widespread anticipation of an imminent Sandinista offensive, informed sources now say guerrilla strategy is an increase in small, debilitating attacks against the National Guard.

In one sense, that tactic appears to be a response to logistical problems, including increased pressure from friendly governments such as Costa Rica and Panama -- themselves under strong U.S. pressure -- to curtail operations on their soil, and heavily reinforced National Guard presence along Nicaragua's borders.