All four entrances to this city, which is wedged between a river and a range of hills 100 miles north of Managua, are barricaded with bricks, piles of trash and small, smoldering fires.

To enter, one must walk past the barricades and into eerily empty streets, filled with broken glass, garbage and the inevitable fires. Gunshots and explosions sound almost constantly.

Occasionally a siren pierces the gunfire as a Red Cross ambulance, with white flags flying and passengers crouching low, drives in from the hospital on the edge of the city to pick up the dead and wounded in the streets.

Most of the fighting has been near the long, narrow city's center, near a park, a cathedral and a Nicaraguan National Guard garrison. Here, on the edges, the city seems deserted and shut up tight.

Out of the alleyways, young men appear, their faces covered with crude masks made of old shirts with eye and mouth holes. Those without masks pull their shirts and cover their faces to the eyes. Most have pistols stuck in their belts. They demand identification of all who enter.

"We are fighting for the 'Frente' here," they say. The Frente is the Sandinista National Liberation Front - Marxist guerrillas dedicated to the overthrow of President Anastasio Somoza.

"Under a capitalist system," they say, "we are never going to have justice."

It is clear whether these young men - students, workers and peasants who appear to range in age from 12 to 30 - are also representing their elders, many of whom either are behind locked doors or have left Matagalpa to stay with family outside. But their guns, they say, were given to them by their parents to fight Somoza's National Guard.

For five days now conditions of war have prevailed in Matagalpa, Nicaragua's third largest city, with 60,000 residents. Much like Belfast, the city has been divided into zones, some controlled by the youths, others by the National Guard. Nation Guard patrols sporadically venture into enemy territory, where rooftops and around corners.

What is happening in Matagalpa, and to a lesser extent in other towns and cities throughout Nicaragua, indicates, that it is becoming more and more likely that the solution to the Nicaraguan conflict will be a military rather than a political one.

There are, to be sure, moderate and political forces at work against Somoza. They are the opponents who instigated the general strike - planned to continue until Somoza's resignation - that began last Friday.

But judging by the militancy and bloodletting sweeping the country as the pro-Somoza National Guard finds itself fighting armed opposition forces in the cities outside Managua, the situation has gone beyond their control.

The political groups fall between the extremes of Somoza and the Sandinistas. While they applaud guerrilla successes against the government, their feelings about a possible Sandinsita-led Marxist government in the future tange from horror to a willingness to allow the Marxists to share power in the "national democratic government" they hope to form.

The trouble is that these groups themselves are divided along a wide spectrum of ideologies and strategies. There are no clearly identified leaders. There are on definiat plans, other than a broad outline of democratic principles, for what will happen on the day they all dream about - the day Somoza leaves Nicaragua.

More seriously, there is no evidence that any of these groups has more than a smattering of support among the masses they suppose themselves to represent. The masses, at least those who are visible and willing to talk, say they support the Sandinista guerrillas and what they perceive as a socialist system.

While a number of peasants questioned in rural areas seem to have little idea what communism or Marxism is, they say it is the guerrillas who have given them action and victories against the oppressive Somoza government. The Sandinistas are the heroes both of the young men on the street of Matagalpa and of the striking bank workers in Managua.

According to U.S. officials, the lack of a well-defined opposition political platform, plan or candidates has prevented the United States from taking a clear position supporting them against Somoza.

The politicians and business leaders in the opposition say that the all-powerful United States is propping up Somoza with its continuation of economic and military aid, and its refusal to openly support them. Without that crucial support, they say, Nicaragua will become another Cuba.

But the United States has said that so far the politicians has given them no person and no program to support.

Informed observers in Managua, where the situation is fairly tranquil despite the large number of shops and businesses closed, say that the organized political opposition has "a 19th century concept of democracy" in which a small minority forms a "government of the people" without consulting the people themselves.

Chief among the opposition groups is the Brad Opposition Front, a coalition of business, union and political groups. Among its more than 15 member organizations are three factions of the opposition Conservative party.

Many of the business groups are now part of the Board Opposition Front. Also within its wide purview are radical and conservative unions, smaller opposition parties like the Social Christians and the Nicaraguan Socialist Party and, at its most radical end, a group of pro-Sandinista professionals, academicians, industrialists and clergy called "The Twelve."

Outside of the Board Opposition Front are a worker-student coalition and other more marginal groups.

All have issued communiques, all have called for Somoza's resignation and all propose the institution of a democratic government.

Still, the basic questions are unanswered. How will a transition government be formed should Somoza leave? Who will lead it, and can the various groups fairly apportion power among themselves? What is to be done with the largely hostile, well-armed National Guard? When will elections be held? There has been no visible attempt by the Board Opposition Front or any of the others to gather popular support outside their own meeting rooms and communiques.

"Nobody here says 'Viva the Board Front' or 'Viva the Conservative Party,'" said one man demonstrating in a Managua bank strike today. "What we say is 'Viva el Frente Sandinista.'"