MOSCOW, OCT. 31 -- One of the most outspoken proponents of Mikhail Gorbachev's reforms has come under heavy fire from Soviet Communist Party leaders, a senior Kremlin official said here today, signaling that the controversy over Gorbachev's drive to revamp Soviet society may be destabilizing his power base.

Boris Yeltsin, the Moscow party chief and arguably the most loyal and prominent of Gorbachev's supporters, was criticized after a speech to a Central Committee plenum on Oct. 21 here, Anatoly Lukyanov, a secretary of the powerful body, told a news conference.

Lukyanov charged that Yeltsin had made "erroneous assessments" and "practical mistakes" in his speech. According to Lukyanov, Yeltsin criticized the style and activities of the party's leading bodies.

Both the ruling Politburo and the Moscow city party are examining Yeltsin's remarks in detail, Lukyanov said. Western diplomats here widely interpreted such an investigation as a formality that could presage Yeltsin's removal as party boss in Moscow and as a nonvoting member of the Politburo.

By the time the plenum had ended, Lukyanov said, Yeltsin admitted that he had made a "political mistake" and "appealed to be relieved of his duties."

According to reports circulating here, the plenum erupted into a verbal battle between Yeltsin and Yegor Ligachev, the number two man in the Kremlin behind Gorbachev, and Yeltsin threatened to resign in protest at the end of the plenum.

Yeltsin is said to have attacked Ligachev for slowing down the pace of Gorbachev's reforms and to have generally criticized the party leadership for half-hearted commitment to the reform program. Lukyanov and other Soviet officials declined today to discuss the details of Yeltsin's speech, however.

The plenum speeches have not been published, and Lukyanov is the first senior official to confirm publicly that Yeltsin is at the center of a major dispute.

Western diplomats in Moscow regard the Yeltsin affair as the tip of an iceberg of controversy over Gorbachev's reform program, with its heavy emphasis on loosening the state's grip on the Soviet economy and encouraging greater criticism of the party and party leaders.

The heated plenum came one day before U.S. Secretary of State George P. Shultz arrived here to set the date for a U.S.-Soviet summit on nuclear arms issues and may have affected the tone of the talks, diplomats here said.

Shultz's visit here ended with Gorbachev declining to give a date for the summit, but officials from both countries agreed Friday to start it on Dec. 7 in Washington.

Lukyanov dismissed reports that the Yeltsin dispute had caused a crisis in the Kremlin. "There are no grounds for rumors that there is no unity in the leadership of the Communist Party of the Soviet Union," he told reporters.

After Gorbachev opened the plenum, 26 Central Committee members spoke, including Ligachev, KGB director Viktor Chebrikov and several other conservative senior Kremlin officials. "Members of the Central Committee did not agree" with Yeltsin, Lukyanov said, adding later that he was among those who objected to Yeltsin's remarks.

By detailing the dispute in a prepared statement, Lukyanov appeared to be indicating that the tide of the plenum ran strongly against Yeltsin and for Ligachev, widely believed to be the Politburo member who is most cautious about the Gorbachev reforms.

Lukyanov, 57, is the Central Committee secretary in charge of Politburo administration and party staff work.

Yeltsin, 56, is a no-nonsense party activist whom Gorbachev brought to Moscow from Siberia two years ago to root out corruption and usher in reforms in the Soviet capital. He has since gained a reputation as the point man on the most controversial of Gorbachev's programs. It was Yeltsin, for example, who first attacked party officials for using special stores and having other privileges not available to ordinary citizens.

In recent weeks, however, Yeltsin has come under criticism for allowing food supplies in the Soviet capital to run low and the quality of transportation and other services to deteriorate.

According to one account of the plenum, Gorbachev's opening speech criticized the management of Moscow for failing to keep up with the required pace of reforms.

Yeltsin reportedly responded by blaming the slow pace of reforms in Moscow on the conservative stance taken by other senior party officials.

Among other officials, he reportedly implicated Ligachev. It is unclear, however, whether he mentioned Ligachev by name.

Ligachev reportedly then took the floor and attacked Yeltsin for poor management, sparking a general debate about Yeltsin and his program.