They are an unlikely collection of heroes; among them an aging industrialist, a dentist, a corporate attorney, two priests, an architect and an agronomist.

"The Twelve" as they are called here although their original number recently shrunk to ten, are an even less likely band of revolutionaries.

Yet, these men, all prominent, but primarily nonpolitcal Nicaraguans, have become a rallying point for the growing opposition movement that has pitted virtually every section of Nicaraguan society against President Anastasio Somoza.

The Twelve hope to unify the far-from-cohesive mixture of conservative and liberal politicians, businessmen, farmers, students and Marxist guerrillas whose only common goal is Somoza's ouster and the speedy dismanling of a 44-year-system of government identified with the Somoza family.

Last fall, the Twelve fled the country after a warrant was issued for their arrest for signing a document supporting the guerrillas. Two and a half weeks ago, after beleaguered Somoza lifted the charges on a technicality, the Twelve returned to Nicaragua.

Since then, they have met here daily, issuing more documents and traveling to massive rallies held in their honor.

There have been a number of major clashes between students and the National Guard, controlled by Somoza, in which at least 12 persons have been killed.

A nationwide, strike against Somoza has been conducted by opposition political leaders. Last weekend, the Managua offices of opposition newspapers were riddled with machine-gun fire twice. Three days ago, according to a government statement, two persons were killed and five wounded in a shootout following a guerrilla rocket attack on Somoza's presidential offices.

The Twelve say their objective, is to "somehow save lives, to mitigate the violence," while unifying the disparate groups calling for Somoza's resignation.

Last Sunday, an estimated 4,000 people turned out for a rally honoring the Twelve in Esteli, a city of 18,000 in Nicaragua's northern cattle-raising country.

The rally was conducted under the watchful eye of the National Guard, Nicaragua's joint army and police force, stationed on a rise overlooking the town's entrance.

There was no intervention.

In many ways, Somoza's hands are tied in repressing rallies like this by the growing squeamishness of the United States, his oldest and strongest supporter.

Recent U.S. policy toward the rigidly anti-communist president has occasionally been marked by a schizophrenic withdrawal of aid, followed by pats on the back promoted by Somoza's many friends in Congress. Carter administration sources indicate that U.S. policy is publicly neutral, but morally anti-Somoza.

It was heavy U.S. pressure, one of those sources insisted recently, that led to Somoza's lifting of a four-year state of siege last fall, including the removal of press censorship.

"We have told Somoza," the source said, "that if he reimposes the state of siege, closes the opposition paper, or arrests opposition political leaders, "the U.S. ambassador will be recalled, and we might break relations."

The United States seems to have no contingency plans for dealing with a new Nicaraguan government led by the Marxist opposition faction should the post Somoza transitional coalition proposed by the Twelve fail. U.S. Ambassador Mauricio-Solaun, however, meets regularly with the prominent opposition politicians and business leaders.

The abrupt turnabout of the traditional U.S. interventionism in Nicaragua seems primarily based on embarrassment at the long economic and political support for Somoza and the National Guard. "We are not intriguing against any opposition faction. The fact is, we're against Somoza," said the Carter administration source.

The opposition charges Somoza, who was preceded in the presidency by his father and brother in a line rarely broken since 1933, with rigging elections and keeping the country under iron-fisted control through an economic and political patronage system and the 7,500 member National Guard.

But according to one diplomatic observer here, "Somoza's main problem is greed."

While he denies reports that his personal fortune, which includes a substantial percentage of Nicaragua's land and industries, totals more than $500 million, Somoza recently confirmed that it amounts to at least $100 million.

Somoza describes himself and his father, who built the family fortune from a small coffee plantation, as simply "good businessmen." But it has been his ostentatious accumulation and display of wealth, at the expense of both competing local capitalists and the poor, that has polarized the country.

In a recent interview at Somoza's Managua office located inside a National Guard garrison which his friends and foes both call "the bunker," Somoza characterized his opposition as "a minority" composed of "irresponsible children," indoctrinated peasants, jealous businessmen and the "extreme left."

The Twelve, he said, "try to sell democracy, but when you go down to the party line, they are all generally communists."

The Twelve deny this, and their known political affiliations stretch from the unconcealed socialist leanings of the Jesuit priest, Fernando Cardenal, to the previously unrestrained capitalism of industrialist Emilio Baltodano and Joaquin Cuadra, 60, one of Nicaragua's best-known corporate attorneys.

Over a low-calorie "bunker" lunch of squash and chicken-falvored barley that is part of his strict died since a serious heart attack last year, Somoza said he "would be crazy" if he did not acknowledge that the Twelve have some support.

As for their demands that he resign and leave the country, Somoza said, "well, that's their privilege."

Determined to complete his six-year term, scheduled to end in 1981, Somoza said his job is to "uphold the constitutional conventions" of Nicaragua. "Otherwise this country would go to hell," he said.

"I'm a hard nut," he said. "They elected me for a term, and they've got to stand me."

Although the extent of public opposition to Somoza has grown enormously and has become more radicalized over the past several months, the Twelve believe Somoza himself is so isolated from the tenor of the country that it may literally take a stick of dynamite to budge him.

While guerrilla leader Plutarcho Hernandez assured a Mexico City press conference recently that "the dictatorship is at the point of collapse," Somoza said he expects things to "quiet down."

Central to this belief is Somoza's often-stated theory that everyone is Nicaragua has his price. The businessmen, he said, have already seen their profits falling as a result of the chaos, and are pulling back.

At the same time, Somoza plans to launch a series of social reforms and public works projects in hopes of buying off the peasants and convincing them they have more to gain from stability.

But the reforms may be too late. The streets and buildings of many Nicaraguan cities and villages are papered with opposition posters and graffiti, and Nicaraguans of all opposition sectors publicly denounce the government in terms that last year would have been unthinkable. Recently, a week's tour of several rural and urban areas failed to produce even one "Viva Somoza" scrawled on a wall.