Perhaps the clearest indication of a new mood in this country is a voluminous flow of letters to Soviet newspapers.

During the past few months such letters have become unusually outspoken and full of surprises. They talk about "dishonesty" of unnamed leaders, or advance such heretical ideas as the need for abolishing centralized planning.

The country's largest weekly newspaper, Literaturnaya Gazeta, recently summarized its weekly mailbag by saying that "as we can see, the authors of many letters are advocating a system of economic relations which would be significantly different from the existing one. The letters also talk about another thing, namely that a new type of economic thinking is in the process of formation."

Letters published in the Soviet press probably carry more weight than those that appear in American newspapers. This is partly because of differences between a free society and a closed society. A local official seeing a complaint against him published in Pravda is likely to think twice before dismissing the grievance again. Pravda, after all, is the official paper of the ruling party.

Another reason, however, is historical. For centuries any Russian with a grievance sought to take his case to the czar. In the early days of the empire, a special basket used to be lowered at regular intervals from a window in the old Kremlin palace to collect petitioners' letters.

The tradition continues to be practiced in a new form. For residents of Moscow, it can be done by dropping petitions in a special box at the Communist Party Central Committee headquarters. For most of the population, however, the favored route is to write a letter to Pravda, Izvestia or other newspapers.

In recent years, the party has regarded letters as an important safety valve and a way of allowing public opinion a legitimate and quasi-independent outlet. Moreover, the letters play a vital role in assessing the mood of the country and keeping the leadership informed about social and economic grievances and ways to solve them.

The volume of letters is enormous. Pravda and Izvestia, the two principal newspapers, receive between 9,000 and 11,000 letters from readers each week. The mailbag of other Soviet newspapers ranges between 10,000 and 13,000 such missives per month for each.

By law, every letter has to be answered within 10 days. Because of their official status, the newspapers have the right to carry out investigations and subpoena and inspect any official documents they want to check, including police and Communist Party records.

As an indication of how seriously the letters process is viewed, the largest editorial section in any Soviet newspaper is the letters department. Izvestia, for instance, has 65 staffers who read letters. Each letter is supposed to be read.

The letters are classified by topics and forwarded to relevant ministries. A small number deemed "typical" by the staff are published. The answer most frequently received by petitioners is a card acknowledging receipt of the letter.

As a result, the letters pages serve a dual function. On the one hand, they provide the Kremlin with information about public opinion while, on the other, they allow it to shape that opinion by carefully and deliberately selecting how much information is to be doled out to the public--the process that allows the authorities to define the issues and cast them in a way of their choosing.

Some newspapers have established a reputation for championing their readers' causes by sending reporters to investigate typical grievances ranging from unfair punishments inflicted by obtuse local leaders to poor services, instances of corruption and other personal problems.

Some newspapers have also featured an "agony column" aimed at young people whose letters reflect confusion about new social and sexual attitudes. Typical is a sad story recently of unwanted pregnancies in a high school in a small Siberian town. The newspaper Komsomolsakya Pravda investigated the case and denounced the school's sanctimonious condemnation and ignorance of sexual activities.

A novel feature in the letters pages, however, is a new focus on large and previously taboo subjects.

Pravda, for instance, printed a letter last week written by G. Varina, a factory worker in Chita, in Siberia. She talked about "dishonesty" among leaders, poor working conditions and official "lies" about production goals.

"Everything is allegedly going fine," she wrote. "But that is a pure lie. We have to live with the truth no matter how bitter it may be. The lies are a corrosive which undermine the human spirit and destroy the people."

A letter written by Y. Fyodorov from Varkuta, in the far north, urged the authorities to do away with the centralized planning system. "Planning should be done by individual enterprises and be based on consumer demand," Fyodorov wrote. A writer from Moscow, V. Agayev, proposed a new incomes policy based on economic performance and "rewarding those people who seek out customers and better meet their needs."

A young woman from Kalmna, in the Moscow region, described in her letter how students are being drafted to join the Komsomol, the Young Communist League. She said, "We simply received a call from the local office of Komsomol to immediately send 15 persons to join the organization." This presumably was to allow the local office to meet its quotas.

A group of university students denounced the educational system, saying they are learning very little and that they are studying in "a terrible atmosphere that fosters passivity and silence" and prevents them from becoming "self-reliant and independent" persons.

Yet another student said in her letter that although she was a member of the Komsomol she believed in God. "I do not see anything bad in this," the student, 16, said. "But if I say that I believe in God, they will expel me from the Komsomol. Why?"

Soviet journalists say that the newspapers have received similar letters in the past but they were not printed. Traditionally no measures are taken to intimidate those expressing dissenting views, presumably because the mailbag is one of the few reliable means the party has to gauge reactions to its policies.

The fact that such letters are printed reflects an attempt by the new Kremlin leadership to mobilize public opinion behind its plan to revitalize the Soviet economy and society. Such public support may be necessary, at least in part, to overcome bureaucratic resistance to changes.

In retrospect, it may be significant that one of the first published Politburo statements since Yuri Andropov became Soviet leader last November was devoted almost entirely to letters. The document, published Dec. 11, said that the letters should be regarded as "political activity of the working people and their direct participation" in the improvement of economic management system.

"The letters are the instruction of the working people to the party and Soviet organs," it added.