Marshal Sergei Sokolov, 73, a longtime military man who was a front-line tank commander in World War II, was named today as the new Soviet minister of defense.

He was appointed by the Presidium of the Supreme Soviet to replace Marshal Dmitri Ustinov, who died Thursday at age 76.

The selection of Sokolov, who has served as first deputy defense minister for the past 17 years, appeared to be a transitional step reflecting reluctance on the part of the Kremlin old guard to appoint a younger man to one of the main positions of power here.

Diplomatic observers also saw it as signaling further consolidation of President Konstantin Chernenko's authority.

It was not clear whether Sokolov would be promoted to the ruling Politburo. He has been a full member of the Communist Party Central Committee since 1968.

Ustinov and his predecessor, Marshal Andrei Grechko, were full Politburo members. The combination of the two positions made them not only top administrators of the country's defense establishment but also key players in shaping Soviet security policies.

The elevation of a well-known if colorless career soldier to the top defense post was presumably calculated to please military professionals without threatening the political leadership's supremacy over the armed forces.

There had been speculation here yesterday that Politburo member Grigori Romanov, 61, a civilian whose career pattern is similar to that of the late Ustinov, might be tapped for the job.

Romanov was named head of the committee organizing the funeral of Ustinov. At the time of Grechko's death in 1976, Ustinov had been named chairman of the funeral committee and was appointed defense minister immediately after Grechko's funeral.

That the leadership chose Sokolov appeared consistent with the basically conservative style of Chernenko.

Chernenko, 73, was visibly shaken today as he and other Politburo members formed an honor guard around Ustinov's flower-decked coffin at the Hall of Columns in downtown Moscow.

As two days of official mourning began today, thousands of officers, soldiers and civilians filed past Ustinov's coffin. Ustinov will lie in state until Monday, when he will be given a hero's funeral and buried in the Kremlin wall along with other prominent communist figures.

The departure of Ustinov from the political stage reduces the number of top policy-makers around Chernenko and strengthens the positions of the Soviet leader's personal assistants and advisers.

One indication of the latter point is provided by recent official photographs in which key personal aides of Chernenko appeared standing next to the members of the Politburo and secretaries of the Central Committee.

Diplomatic observers here said Sokolov's appointment was not likely to have any significant impact on Moscow's foreign and security policies. Its import, they said, is rather to suggest that the present leadership seemed inclined to postpone the ascension of a new generation, something that Yuri Andropov, Chernenko's predecessor, had advocated vigorously.

Unless he is named a member or candidate member of the Politburo, Sokolov's voice in Soviet decision-making will be that of an expert adviser and executor, rather than a policy-maker.

The new defense minister is the oldest man ever to be appointed to the post here. He is basically the type of soldier whose views were shaped by World War II. After the war he commanded two important military districts and was unexpectedly vaulted to the post of first deputy minister under Leonid Brezhnev in 1967.

Western military specialists said he was the chief administrative officer in the ministry, and kept a low public profile. Unlike Ustinov, Sokolov has no distinct political record.

He was born July 1, 1911, at Yevpatoria, in the Crimea, the son of an ethnic Russian white-collar worker. He joined the Army in 1932. Five years later he joined the Communist Party.

He specialized in armored warfare, graduating from a tank academy and rising to command a battalion. During the war he held command positions with Soviet tank forces, mostly on the Karelian front in northwestern Russia. He was a colonel when the war ended.

After the war, Sokolov attended two military academies, the first specializing in tank warfare and the second a general staff college in Moscow that set him on his path upward. He was a two-star general when he headed the Moscow military district general staff from 1960 to 1964. He was promoted to three-star general in 1964 when he assumed command of the Leningrad military district.

His promotion to first deputy defense minister came in 1967, when he also became a four-star general. A year later he was made a full member of the policy-making Central Committee and in 1978 he was promoted to the rank of marshal.

As one of the three first deputy ministers, Sokolov seemed to be deeply involved in Soviet arms sales. In that capacity he traveled extensively in the Third World.

Given his age and background, Sokolov appears to have been the least controversial choice to succeed Ustinov.

But the appointment raises several issues on the domestic front. One is the question of modern weaponry. Ustinov had a formidable grasp of defense production and was the prototype of the Soviet technocrat, whose skill and experience allowed him to administer the modernization of the Soviet Union's armed forces.

According to well-informed sources, it has been privately argued here that the country needs a younger man with an engineering background to compete successfully with the United States in the production of new weapons systems. Although Sokolov has had a great deal of troop command experience, according to this reasoning, he may not have the sort of grasp of modern weapons technology that Ustinov had.

Another problem caused by Ustinov's death is the balance of forces in the ruling council. The Politburo now has 11 full members, and it is expected that Chernenko may bring in one or two new ones at the next meeting of the Central Committee.

But the death of Ustinov, whose prestige in the party and the country was great, seems a substantial loss in terms of domestic politics, particularly at the time of a new Kremlin succession.

Since 1973, the Soviet leadership has recognized the importance of officials who head the Defense Ministry, the Foreign Ministry and the KGB secret police. This suggests that Sokolov might be promoted to candidate member of the Politburo.

But even if elevated to the Politburo, Sokolov is not expected to play the kind of role that Ustinov, Andropov and Foreign Minister Andrei Gromyko played during the difficult transition that has been under way since the death of Brezhnev in November 1982.