Nicaragua's leading newspaper, Le Presna, while still strongly supporting the five-week-old Sandinista government it helped to put in power, has now begun to assume what its editor calls its rightful function of "constructive criticism."

The paper's reappearance in a mildly critical role and the reopening last week of private radio stations have provided comforting signs for Nicaraguans and foreigners watching for progress toward a more democratic government than that of ousted president Anastasio Somoza.

The praise for the new government is still there -- pages of pictures and stories regularly glorify wartime Sandinista achievements -- but now there is equal emphasis on reporting the government's problems, abuses of power and a wide range of international news drawn from Western wire services.

In an editorial at the weekend, La Prensa said pointedly that "when there is time to correct errors, there should be no time to invent excuses."

The paper, whose offices were destroyed by Somoza's National Guard during the civil war, criticized the delays faced by thousands of people who must wait in line to get visas.

"If there aren't enough personnel to attend to them, there are plenty of people looking for work in this country," it said.

The paper also criticized the Sandinista military for tying up traffic throughout the city and accused some government officials of delaying programs by considering themselves "indispensable" and not delegating enough authority.

Following the announcement of a new press law guaranteeing free expression, radio stations that had been hooked up to the national Sandinista news network went on their own. Some now pick up government programs. Others do not. Most have returned to their pre-revolutionary diet of U.S. pop hits and Latin love songs.

The government has announced, however, that all radio stations and Nicaragua's two television channels are "under investigation" to determine which of them belonged all or in part to Somoza, members of his government or strong supporters who fled the country.

All such Somoza-connected property has been expropriated. One of the two television stations, belonging to Somoza himself, has already been taken over by the state. Two radio stations -- Station X, belonging to Somoza, and Radio Managua, property of a member of Congress from Somoza's political party -- have also been nationalized.

The government has been meticulously going over ownership papers of every company and piece of property in Nicaragua to determine what now belongs to the state under the expropriation decree.

A Justice Ministry spokesman said that the ownership question was the only governmant concern in the matter and that those stations found unaffected by the decree would not be interfered with.

Typical of the more relaxed attitude, even Radio Sandino, the official "Voice of the Revolution" now spices its communiques and hours-long broadcasts of political events with Barry Manilow and the Bee Gees.

"Everybody likes the revolutionary hymns," one housewife said. "But you get tired of hearing "The Tomb of the Guerrillas" -- one of the Sandinistas' most sacred songs -- "20 times a day."

The new press law contains none of the restrictions and provisions for government-imposed censorship and shutdown in what journalists here called the "black code" of the Somoza government.

It does, however, order that publications display "a legitimate concern for the defense of the conquests of the revolution, the reconstruction process and the problems of the Nicaraguan people" and serve as "vehicles for the development and cultural and educative process" of the country "when necessary."

It mandates that "critical commentaries on [government] functions, as well as all news, should be based in constructive ends and provable facts with which those responsible have been objectively confronted."

Liquor and cigarette advertising is prohibited, as are the use of names or pictures of revolutionary "heroes and martyrs" or anything associated with the "revolutionary struggle" for commercial purposes. There are prohibitions against invasions of privacy and exploitation of "women as sexual or commercial objects."

As in the United States, ownership and regulations of the nation's airwaves are delegated to the state.

The Sandinista National Liberation Front is still represented by its own newspaper, Barricada. Once distributed free, it now costs 10 cents a copy.

Bannered daily with the red and black Sandinista colors and printed on the expropriated presses of Novedades, the paper owned by former president Somoza, Barricada is an unblushing propaganda organ of moderate Socialist politics and Third World news.

But even at 20 cents a copy, La Prensa has more than twice the circulation of Barricada.

La Prensa reappeared Aug. 16 for the first time since its printing plant was pounded by National Guard tank and artillery fire before being set ablaze with gasoline.

With its offices and presses now a rusting pile of charred rubble on a concrete foundation, La Prensa is put together and printed in a complex system of round-the-clock activity.

The 12-page paper has changed from an afternoon to a morning daily, since the only press available to print it is located in Leon, 54 miles north of Managua. Working with two telephone lines in a tin-walled warehouse behind their former building, La Prensa's staff puts the offset pages together and speeds them north by road at 5 p.m.

Member newpapers of the Interamerican Press Association have donated spare equipment, including typewriters and cameras, and larger firms, such as the gift of a photocomposit machine from El Dario de Hoy, one of El Salvador's two conservative papers.

The in-house union representing La Prensa's 180 employes has agreed to work for 65 percent pay for the next several months, but director Xavier Chamorro said the paper's biggest problem is still money.

"We have nothing," he said, "no working capital, no money to buy new equipment."

However, both private Nicaraguans and the government, conscious that freedom of the press is one of the things on which the new system will be judged, pushed for the paper to resume publication, even on a shoestring budget.

Xavier Chamorro, who replaced his brother Pedro Joaquin Chamorro as director of the paper following the latter's murder in January last year, said La Prensa has no intention of opposing the Sandinistas or questioning the foundations of the revolution.

The paper, in fact, was one of the principal supporters of Somoza's overthrow and was many times censored or shut down, and finally destroyed.

Still, Chamorro said, "liberty of expression and opinion are as valid today as they were yesterday. We were responsible in hard times, and we're trying to be even more responsible now. Our role is to serve as a healthy, constructive critic."

Because Pedro Joaquin's widow, Violeta, is now one of the five-member junta government while still owning part of the paper and serving on its board of directors, La Prensa has a certain conflict of interest, at least by U.S. standards.

Xavier Chamorro maintained, however, that his sister-in-law has nothing to do with the editorial side of the newspaper, which he described as "completely autonomous."

Chamorro said he expected the newspaper to expand its investigation of government problems and its citizen complaint center. Even though it is still difficult to reach La Prensa on the telephone, a number of people turn up at the office daily to complain about something.

Now their favorite topic, apart from the search for missing relatives, is the lines for exit visas.

While the government says it realizes the problems of getting out of Nicaragua give the impression that exit is restricted, it maintains the difficulities are bureaucratic -- due to the large number of people who have been stuck here throughout the war and now want to get out even temporarily for business or personal reasons -- and a result of destruction of documents by the old government.

But as Pedro Joaquin Chamorro, Jr., who writes for La Prensa, said in his editorial column Saturday, if its a question of wanting to keep former government henchmen or anyone else from leaving, "anybody who wants to leave can do it without the emigration office. This country has 500 miles of borders and even more seacoast."

During the war, he said, "the Sandinistas themselves were a testament to how easy it is to get in or out."