A senior member of the Kremlin's old guard was retired today as the Communist Party boss of Moscow, a key step in Soviet leader Mikhail Gorbachev's reorganization of the top ranks of the party.

The Soviet news agency Tass reported that Gorbachev was present today at the plenary meeting of the Moscow party committee that "relieved" Viktor Grishin, 71, "due to his retirement."

With this unceremonious departure, Grishin, one of the longest serving members of the Politburo, relinquished his political power base, apparently yielding to forces lining up behind Gorbachev as February's crucial 27th party congress approaches.

With preparations for the congress now at the halfway point, western and Soviet sources are predicting that at least half the party's Central Committee members, numbering about 300, are likely to be replaced. The Central Committee is the core of the Soviet Union's ruling elite and is considered the key to Gorbachev's efforts to improve the economy.

Grishin's departure from the Moscow post is expected to be followed by his resignation from the 12-member ruling Politburo. He has been replaced as Moscow party chief by Boris Yeltsin, a secretary of the Central Committee who was elevated to the post by Gorbachev last July.

A recent series of articles critical of the management of Moscow affairs gave a strong indication of Grishin's tenuous hold on his job. Last October, Grishin had to take the unusual step of defending himself in the press against charges that dishonest practices in Moscow's housing program had wasted thousands of rubles.

Grishin was one of the last remaining members of the Politburo's older generation, an ally of the late Leonid Brezhnev. According to some accounts, he may have resisted Gorbachev's accession.

At the least, that political profile indicated that Grishin was not in step with the new team that Gorbachev is actively assembling throughout the country.

Some diplomatic observers today speculated that Grishin's departure could mean that other members of the Politburo -- particularly Dinmukhamed Kunaev, first secretary in Kazakhstan, and perhaps Vladimir Shcherbitsky, first secretary in the Ukraine -- could also be on their way out.

As the party's congress approaches, the press is filled with accounts of personnel changes at the local party level, in the ministries and in the Presidium of ministers. Less visible are changes further down in both the party and government apparatus, where some say thousands of people have been retired, shifted or, in some cases, pushed out of Moscow-based jobs.

As another example of the rapid pace of change, western business sources reported today that they had learned that deputy foreign trade minister Vladimir Sushkov had been relieved of his duties. Sushkov was cochairman of the U.S.-U.S.S.R. Trade Council and was well known among American businessmen.

The head count of the known changes has become increasingly complex, as the tempo picks up. But according to one count, the score card, assembled since Gorbachev's accession in March, shows a turnover in 22 of the more than 80 positions of ministerial rank, including heads of state committees; 40 of the 157 first secretaries of regional party committees, and four of the 15 first secretaries of the Soviet republics.

Many of the changes have taken place gradually in recent months at special local party meetings. More are being added now as the regional committees hold their election campaigns as part of preparations for the congress.

According to one count, more than 90 of the regional party committees have yet to hold their precongress conferences. Of those, turnover at the first secretary level is possible in more than 70.

By mid-January, the republics will start holding their conferences to elect delegates to the congress. Historically, Moscow, which with 8 million people is by far the country's largest urban center, has held its city party conference at the same time as the republics, a sign of the importance the city holds in party affairs.

Grishin's retirement came after Moscow's local committees concluded their meetings this week, timing that suggested to some diplomats that Gorbachev wanted to get his Moscow team in place before the city conference convenes.

The timing was reminiscent of the decision last October to retire Nikolai Tikhonov as Soviet premier a week before the party's Central Committee was due to meet for a plenary session and two months before the Soviet parliament met in ts regular session, the time when the change should have taken place, according to strict legal interpretation.

Some political observers have suggested that these early moves show that Gorbachev still faces resistance among the party elite. By presenting personnel changes as a fait accompli, he risks less of an overt challenge at the party meetings, according to this theory.

Still, Grishin's exit has been long expected, particularly after the spate of articles criticizing the city's administration for failures and embarrassing shortages in the housing industry and, more recently, for failing to have roads ready for the grueling Russian winter.

Diplomats and Soviets said they expect that Grishin's retirement will lead to change in Moscow's city hall, where Vladimir Promyslov, Grishin's close associate and Moscow's mayor for more than 20 years, also has been cited in connection with failings in city services.

Grishin, who first became a candidate member of the Politburo in 1961 and a full member in 1971, was as close as the Soviet Union comes to a party boss. In recent months, it became apparent that he would be left behind by the Gorbachev bandwagon.

The public criticism of Moscow authorities began with articles saying that the city had tinkered with figures to show that it had met its housing targets. While the practice was not unheard of elsewhere in the Soviet Union, the newspaper Sovyetskaya Rossiya said, "Negative phenomena are particularly intolerable in the capital."