Six months after coming to power, Soviet leader Yuri Andropov appears to have established a strong grip on many of the levers of power in the Kremlin. But Andropov's physical frailness, his diffident public manner and his apparent determination to build consensus rather than to rule his Politburo colleagues by fiat have left a surface impression of tentativeness to his rule.

Walk into a western embassy in Moscow today and talk to junior officials, and you are likely to be told of a raging power struggle within the Kremlin, with an embattled Andropov barely fending off challenges from proteges of the late Leonid Brezhnev. Walk a few offices farther down the corridor in the same embassy, and more senior diplomats will tell you that there is nothing to such reports.

Moreover, usually well-informed Soviet observers, who correctly reported a power struggle last year as Brezhnev was dying and Andropov was moving to replace him, are saying now that those divisions have subsided. And the openness with which official Soviet newspapers and magazines are discussing the need for Andropov to advance a comprehensive program to match his ambitious initial pronouncements suggests a popular acceptance of him as the man in charge.

In a society where it is frequently said that the ruler has to pound the table to be heard and obeyed, Andropov's voice does not yet appear to have penetrated to the factories and farms in the vast regions of the Soviet Union. Here in Moscow, however, the man who headed the KGB for 15 years appears to have all the power he needs to make the Politburo fall in behind him.

Andropov's name now dominates the Soviet media the way newly erected billboards with quotations from his speeches dominate Moscow's major thoroughfares.

In his rare public appearances, he seems diffident and camera-shy, a complex and somewhat hesitant leader whose style is different from that of his predecessors. But foreigners who have dealings with him describe Andropov as being incisive and in command of facts and arguments and as a man who knows what he wants and why.

But domestically, impatience with that public tentativeness appears to be growing. Despite his clearly stated intentions to assign highest priority to restructuring the Soviet economy, his government has failed to advance a comprehensive program of its own.

Reports in March that Andropov had been hospitalized with an unspecified kidney ailment triggered much of the talk abroad about a possible power struggle. Andropov quickly recovered and is portrayed by the media as maintaining the fast pace set during his first three months in office.

Next month, Andropov will be 69. Assuming a four- to six-year span in power, he would have a much shorter span than his predecessors for making his imprint on Soviet society.

But the circumstances under which he came to power define a large measure of the authority he has already asserted. After spending 15 years as the head of the Soviet secret police and then moving back to the Communist Party's Secretariat only six months before Brezhnev's death, Andropov was the candidate of the military and the security apparatus at a time of growing international tensions and following a series of setbacks for Moscow in Poland, Afghanistan and in the economy.

This enabled him to turn back a challenge by Brezhnev's supporters, who backed Konstantin Chernenko, 7l, Brezhnev's closest political associate. At the critical meeting of the entire Soviet leadership following Brezhnev's death, Defense Minister Marshal Dmitri Ustinov not only put forward Andropov's name but also reportedly made it clear that Chernenko was not in favor with the Soviet armed forces chiefs.

The succession brought to the fore the three institutions responsible for internal and foreign security affairs, whose importance was formally recognized by Brezhnev in 1973 when he promoted to full Politburo membership the heads of the Defense Ministry, Foreign Ministry and KGB.

It is no accident that the key men in Andropov's leadership are Ustinov and Andrei Gromyko, who added the responsibilities of first deputy premier to his existing foreign minister's portfolio last month, along with Ukrainian leader Vladimir Shcherbitsky. They backed Andropov against what was said to have been a pro-Chernenko majority on the Politburo, the party's supreme policy-making body.

Chernenko, who has been absent from public view for more than a month now due to what his office describes as a case of "mild flu," has shown every sign of being reconciled to Andropov and accepting his position as a senior Politburo member.

This has meshed with Andropov's style of working through consensus rather than riding roughshod over the sensibilities of colleagues or opponents. While he has carried out changes in the governmental apparatus, he has refrained from making changes in the party's leadership. There is one view here that precisely because of his KGB background, Andropov wants to avoid giving the impression that the Kremlin is in the hands of ruthless security and military establishments.

The new leadership is promoting wide-ranging public discussion on the question of economic changes. It is continuing its anticorruption drive, which involves some of the highest figures in Brezhnev's government. Among those being investigated is Gen. Nikolai Shcholokov, who served as interior minister for 16 years until he was ousted recently.

Because Shcholovkov remains a member of the Central Committee and a deputy of the Supreme Soviet, he cannot be arraigned or arrested. But well-informed sources say members of his immediate family are under investigation on corruption charges.

Lower-level Soviet officials and western diplomats expect the questions of the economy and of the still vacant post of chairman of the Presidium of the Supreme Soviet, or head of state, to be dealt with in mid-June at a plenum of the Central Committee. The plenum will be followed by a session of the Supreme Soviet, or legislature. Until now, it has been unclear why Andropov has left vacant the largely ceremonial head-of-state position that was occupied by Brezhnev in his later years.

Soviet observers suggest that Andropov is a methodical person and add that western diplomats and Kremlinologists in foreign capitals who interpret his caution as a sign of a power struggle are mistaken.

The prevailing view among foreign and Soviet observers here is that Andropov is in full command at this stage and that he can get his program through the Central Committee. Resistance to changes is thought more likely to come from the ranks of the vast party bureaucracy spread throughout the country.

So far, the new leadership's main theme has been a call for greater social and labor discipline, and official figures show that this drive has produced significant results. In the first three months of this year, industrial production rose 4.7 percent, and productivity was up by 3.9 percent over the the first quarter of 1982.

Discipline by itself, however, cannot sustain economic performance over the long term, a fact underscored by the grass-roots grumblings that have followed the initial popular welcome of Andropov's campaign.

Press accounts make clear that the discipline drive has to be supplemented by new policies providing the necessary incentives for people to work harder and more efficiently.

The clearest signs of popular mood in response to Andropov's campaign are reflected in letters to the editor, the sheer volume of which suggest that not only has it touched a popular chord but also that Soviet citizens are grasping a new opportunity to pour out their chronic grievances and complaints.

Poor transportation and shortage of consumer goods are among the continuing complaints. On the positive side, there have been many suggestions on how to increase labor efficiency, including firing lazy workers and financially rewarding more eager employes for increased productivity.

A metal worker from Norilsk, for example, argued in his letter to the daily Trud that difficulties at his plant are due to "poor organization." The workers, he said, would produce more if they were paid more. Another worker in the same city said he had problems obtaining "special clothing for outdoor work" and consequently could not be expected to do more than he was already.

Above the grass-roots level, major newspapers are conducting round-table discussions with economists, managers and workers that invariably are critical of the present state of the economy.

According to the transcript of one such discussion, economist Abel Aganbegjan of Novosibirsk asserted that the Soviet economy must be "reorganized" if it is to "introduce new methods" of management. Aganbegjan criticized "the absence of commercial instincts" in the economy and said industrial managers must be given "greater responsibility for running their enterprises" to be able "to do more for the society."

Aganbegjan also criticized the vertical character of the Soviet command economy and said the key question is to introduce the principle of "supply-demand" if production is to be stimulated.

Another complaint about the planned economy came from the party chief in the small town of Shchekino, who said that central planners in Moscow had suddenly decided to change the production plans for the Shchekino plants. In the middle of the year, he said, "our enterprises were forced to change 20 percent to 40 percent of their production." Such shifts involve changes in prices, additional raw materials and salaries and create havoc, he pointed out.

While these and other economic proposals are being advanced daily in the press, one important issue with which Andropov will have to deal is completely ignored: the problem of shaping his succession, which, next to the economy, he considers as deserving the highest priority.

There is no expectation of a public discussion on this issue, although Andropov has hinted broadly that he would like to have more people from the non-Slavic nationalities in top leadership positions.

On the touchy subject of nationalities, he went to the heart of the matter by publicly suggesting that not all was well in relations among the various nationalities, a situation, he said, that required greater sensitivity in handling the difficulties.

His straightforward approach and the outpouring of response from the public explain why there are signs of nervousness in sections of the party and governmental bureaucracy, which are naturally worried by such a populist approach.

At the same time, it also seems clear that the new leadership is trying to generate pressure from below to keep the momentum for change going.

Whether Andropov and his colleagues can sustain this pace and whether he will be able to carry out his plans remains to be seen. In the course of the first six months, two things have become clear. First, the Brezhnev era is being forgotten with astonishing speed, even though Andropov occasionally quotes from Brezhnev's old speeches on foreign policy and keeps his photograph in his office. Second, the true character of the Andropov period has yet to be revealed; there is still a tentative quality about it.

As the chief editor of Pravda wrote recently, "Nobody is talking about any new rights or freedoms--people are asking, demanding that order be imposed."

It is a curious statement, for it reflects the old Russian problem. The great monk Nestor, in his chronicle written nearly 900 years ago, put it this way: "The land is large and rich, but there is no order."