When counterrevolutionary guerrillas passed through the little village of Muy Muy last week, residents stood by and cheered. Then a few days later, when Nicaraguan militiamen moved in, the same villagers came out to cheer again.

"They must be the most neutral people in the world," cracked a Sandinista official relating what had happened in the hamlet in eastern Matagalpa province.

His story was meant to be humorous, and it described only one incident in a series of clashes. But it went a long way to explain the recently intensified concern here over U.S.-backed antigovernment forces that have been trying in earnest since last summer to overthrow the 3 1/2-year-old Sandinista rule.

During the past few weeks, organized counterrevolutionary bands have for the first time been able to establish a scattered military presence and mount sporadic raids in Matagalpa province, only 70 miles from Managua and nearly 100 miles from the Honduran border mountains where they have rear bases and supply sources.

Sandinista officials estimate the number of guerrillas inside the country at about 2,000, with at least several hundred in the Matagalpa hills. Although they said the infiltrators had been surrounded and then crushed, the Matagalpa raid nevertheless marked the first time the counterrevolutionaries had been able to remain in any numbers away from the northern border region, where they can cross back and forth into havens in Honduras, or the isolated reaches of eastern Zelaya province, where a restive Miskito Indian population provides a friendly environment.

What this means, according to Sandinista officials and foreign diplomats alike, is that the antigovernment guerrillas enjoy at least tolerance, if not support, from some farmers and villagers in the region. For a government that came to power on a wave of popular enthusiasm for its own uprising against the late Anastasio Somoza, the realization amounts to a troubling blow.

It also comes at a time when relations seem to be hardening between the Sandinista leadership and its opponents in private business and the Roman Catholic hierarchy. As a result, there is speculation here among some Sandinista officials that the revolutionary leadership could soon be pressured into harder line political controls that would further taint its declared policy of pluralism.

Pope John Paul II's visit here at the beginning of the month, marked by what amounted to revolutionary cheerleading by Nicaragua's top officials during the papal mass, helped crystallize the antagonism. One lay Catholic activist, a strong Sandinista opponent, called the performance "disgusting."

He said the outcome will be to reinforce the authority of Archbishop Miguel Obando y Bravo in his struggle against Nicaraguan priests and religious who have embraced the Sandinista revolution, sometimes bypassing church teachings on Marxism. A letter is being circulated at churches for signatures containing an apology to be sent to the Vatican in the name of the Nicaraguan church, he added.

Government relations with upper level private business were in effect "frozen" even before the papal visit, business leaders say.

"What relations?" asked Ramiro Gurdian, vice president of the High Council of Private Enterprise.

"The confiscations and the march toward goals indisputably opposed to ours continue," said the group's new president, Enrique Bolanos.

A measure of the contacts with the political opposition--which also embraces the business opposition--came Friday with a confrontation between Interior Minister Tomas Borge and Luis Leiva Rivas, head of the political and business opposition umbrella coordinating committee.

Leiva was summoned by officials of the governing junta to attend a meeting the officials described as a briefing to the opposition on the situation created by the recent counterrevolutionary attacks. The officials said the meeting was an effort to make pluralism more effective.

Leiva declined to attend, saying that he was leaving on vacation to Costa Rica and that the committee would choose someone else later to replace him at the meeting.

As he tried to leave the country, however, he was refused permission to board his flight. His passport was confiscated, he told friends, and he was told to report to the Sandinista security headquarters to discuss his case.

Later in the day, he was taken to see Borge, who dressed him down for refusing the junta's efforts to keep the opposition informed, these friends said.

It is difficult for an outsider to judge how deep into the population such antagonisms reach, however, despite frequent griping about economic hardships and shortages in Managua, the capital. More than half the country's 2.7 million residents are under 15 and youthful enthusiasm for the Sandinista revolution appears to a visitor to remain high.

In addition, even among the government's strongest critics here, the idea of a return to Somoza-style rule promoted by former officers from the dictator's hated National Guard finds no support. With that in mind, the Sandinista leadership consistently portrays the counterrevolutionaries as National Guardsmen seeking to return to power with U.S. help.

Barricada, the ruling Sandinista National Liberation Front's official newspaper, published on Wednesday a page of photos and biographies of leaders of the Nicaraguan Democratic Force, the main antigovernment exile group. "The Beasts Will Not Return," read the headline.

Enrique Bermudez, a member of the exile group's directorate and one of its top military leaders, was, the paper said, "known for his direct participation in the corruption of Somoza's rule." Alfonso Callejas, another of the group's leadership, was shown in a photograph smiling alongside Somoza and Turner Shelton, a U.S. ambassador here during the dictator's time.

The Sandinista leadership thus has been particularly incensed at what it says is an attempt by the Reagan administration in recent days to create the false impression here and abroad of an internal insurrection against the revolutionary government rather than the U.S.-backed attacks from Honduran soil that Managua says are occurring.

The Nicaraguan Democratic Force, seen here as a lever for U.S. policy against Nicaragua, has long claimed support from dissatisfied Nicaraguans within the country. Now, the officials note, State Department spokesmen and U.N. Ambassador Jeane J. Kirkpatrick have emphasized the same theme in their comments on the latest clashes.

With that in mind, Sandinista officials have warned against possible new counterrevolutionary attacks in the south, near the border with Costa Rica, and the northeast, where groups of Miskito refugees have been organized into antigovernment units along the border with Honduras.

The worries do not seem to be military for the most part. The several thousand counterrevolutionary guerrillas reported to be operating in various zones inside Nicaragua pose little serious challenge to the Sandinistas' 22,000-man conventional Army, backed by more than 10,000 trained militia reserves and tens of thousands more volunteer militiamen with rudimentary drill under their belts.

And despite several warnings that Honduras risks war by allowing the antigovernment Nicaraguans to use its territory, Sandinista officials and foreign diplomats say such a conflict is unlikely unless one side or the other makes a severe miscalculation.

Also convincing is the lack of military deployment in the Matagalpa area where recent raids have been reported. One soldier in San Dionisio, where counterrevolutionaries attacked last week, was seen yesterday dozing against a wall in the noonday sun, his AK47 assault rifle cradled in his arms and a transistor radio playing soft music.

The antigovernment raids raised other fears, however, because they fit into what is seen here as systematic persecution by the United States designed to frustrate the Sandinista revolution. Few Sandinista officials get very far in a conversation without recalling a history of U.S. support for Somoza. Since then, they repeatedly say, the Reagan administration has allocated $19 million to finance an anti-Sandinista subversion campaign widely reported in the U.S. press.

Against that background, each counterrevolutionary attack, even if minor in military terms, fits into what is defined here as a pattern of harassment that has the weight of the U.S. government behind it. As a result, the Sandinista leaders react with charges of "invasion" that, viewed from the outside, may seem out of proportion to the small-scale raids actually being carried out.

A Sandinista official, recognizing this, nevertheless expressed fear that U.S. emphasis on internal unrest here could be a source of "war hysteria" in Nicaragua, leading the government to "measures that a lot of people will not like." Already, he noted, long lines are forming at gasoline stations in Managua and stations outside the capital often have nothing to sell.