MOSCOW, FEB. 18 -- Boris Yeltsin, a maverick reformer and ally of Soviet leader Mikhail Gorbachev, was ousted today from the ruling Politburo, effectively eliminating the Kremlin leadership's most outspoken proponent of rapid change.

The move, which was expected after Yeltsin had accused senior Kremlin officials of dragging their feet in Gorbachev's drive to reform the country's economy, was viewed here as a victory for conservative forces in the Soviet leadership.

Yeltsin, 57, was fired as Moscow party leader last November and demoted to a job as deputy head of a Soviet construction conglomerate. His removal as a candidate, or nonvoting, member of the Politburo was decided by the Communist Party's Central Committee at the end of a major two-day plenary meeting.

The powerful, 300-member body also promoted Yuri Maslyukov, the new head of the state economic planning committee, and Georgy Razumovsky, the Central Committee secretary in charge of personnel, to candidate membership in the Politburo.

The plenum left Gorbachev's political support largely unchanged, western diplomats said, but revealed the prominence of some conservatives in the Kremlin leadership.

Yegor Ligachev, seen as one of the most outspoken Politburo conservatives, used a speech yesterday before the plenum to attack some of the trends that have flourished in the ongoing campaigns for democratization and glasnost, or openness.

In his remarks, published today in the party newspaper Pravda, Ligachev assailed Soviet youth for its tastes in rock music, intellectuals for their probes into Soviet history, and Soviets in the Baltic republics, Central Asia and Siberia for their turn toward nationalism.

"If there was any doubt about Ligachev's critical views on some of the effects of perestroika {restructuring} and glasnost," a western diplomat said of the second-ranked Politburo member and chief party ideologist, "he put an end to it with this speech."

In his speech before the plenum today, Gorbachev took a slightly different line, defending the review of Soviet history and calling for a party conference on the question of nationalities.

Ligachev said in yesterday's speech that "apathy, subject to the influence of primitive spiritual products, bourgeois morals . . . this is also a part of reality, as is the attraction of youth to nationalistic tendencies, like the well-known events in Alma Ata, Yakutia, and the Baltic republics."

In the past two years, nationalist demonstrations have taken place in the Kazakhstan city of Alma Ata and the Siberian city of Yakutia, as well as the Baltic capitals of Riga, Tallinn and Vilnius.

Ligachev blasted the "spread of primitive music" and "loud bands" as damaging to Soviet youth culture. He called instead for a return to classical and folk music.

Ligachev also warned indirectly against the new critical analyses some historians are making of Soviet history, including events surrounding the Russian revolution. "Now many talk about historical truth," he said. "It's important to carry through in full to the new generation without slander or ill will, not to hide the truth. But the main thing," he said, "is that behind us is the great history of a great people."

Gorbachev, in his speech today, encouraged deeper investigation of history, saying it should be viewed "in all its diversity."