MOSCOW, FEB. 18 -- Soviet leader Mikhail Gorbachev, seeking to defend his reform policies from domestic critics, called today for an "overhaul" of the Soviet political system and a break with a past dominated by centralized authority and the "rust of bureaucracy."

In a speech on the closing day of a meeting of the policy-making Central Committee, Gorbachev's tone often sounded strident and defensive as he lashed out at Soviet critics for adapting to change too slowly and at "rightists" in the West for opposing further arms agreements.

"The point at issue is not, of course, replacing the existing system, but introducing qualitatively new structures" and providing "new content and dynamism," Gorbachev told about 300 senior Communist Party officials gathered from all over the Soviet Union.

The Kremlin leader, in an unexpected move, also proposed to hold a special meeting of the Central Committee to discuss the sensitive issue of ethnic groups within the Soviet Union, which he described as "the most fundamental, vital issue of our society." He surprised some analysts by referring so directly to recent protests in some of the Baltic and Central Asian republics over Moscow's centralized control.

Today's meeting also removed Boris Yeltsin, a key Gorbachev ally, from the ruling Politburo in what was perceived as an important victory for forces skeptical of radical change.

Ever since Yeltsin was demoted from his powerful post as Moscow party chief in November after accusing leading party officials of thwarting rapid change, Gorbachev has tried to fashion a domestic and foreign reform policy that is at once radical in practice and acceptable to as many members of the Kremlin leadership as possible. In order to do that he has had to employ all of his political skills, placating the more conservative figures, such as ideology chief Yegor Ligachev and KGB chief Viktor Chebrikov, without alienating his core of reformers.

In his speech today, Gorbachev minimized talk of any widespread opposition to reform in the leadership, saying that such talk is a "provocative invention," the product of foreign "radio voices" and other "centers of antisocialist provocation" that "wish to sow uncertainty."

While Gorbachev noted that "for the first time in many decades" the Soviet Union is experiencing "socialist pluralism" and public debate, he insisted that "behind all that" there is general agreement among the leadership on the need for, and the direction of, reform.

In June the Communist Party will convene a special conference, the first such meeting since 1941. Soviet and western analysts here say that, while Gorbachev would like to win full support for increased democratization, radical economic reforms and the replacement of as much as one-third of the Central Committee at that conference, more conservative members of the leadership are unwilling to go that far.

Last week the Soviet leader announced a tentative schedule for the withdrawal of Soviet troops from Afghanistan, a traumatic and humbling chapter in Soviet foreign policy that analysts say has caused many government and military leaders great anxiety. The forcefulness of his criticisms of the West in today's speech seemed designed to soothe those anxieties.

In his address to the party leadership, Gorbachev praised the Soviet media for turning the spotlight of glasnost, or openness, on those "resisting and hampering" reform and democratization.

"To this day we meet those who feel the creeps when watching the scope of the processes of democratization," he said of those reluctant to change. Gorbachev said those opponents are worried about their own "selfish interests" and "this is an absolutely unacceptable posture for party members, especially leading ones."

Speaking of the West, Gorbachev said the positive atmosphere and arms control initiatives that emerged from the Washington summit last December go "against the grain with some people."

"Very soon after the first days of euphoria, the opponents of normalizing relations with the U.S.S.R. started 'sounding the assembly,' mobilizing their forces for the struggle against the ratification of the treaty" banning medium- and shorter-range nuclear arms, Gorbachev said.

"The U.S. administration is as good as its word in upholding the treaty. Yet it simultaneously echoes the ultra-rightists in their anti-Soviet, anticommunist rhetoric. And not only in words, but also through certain actions in the militaristic style under the same pretext of a 'growing Soviet threat.' "

As he has in previous meetings with foreign leaders, Gorbachev criticized the "European part" of NATO for trying to "compensate" for the nuclear forces being eliminated under the treaty.

"They are planning a modernization and a build-up of 'other' types of nuclear weapons, especially at sea and in the air, cynically claiming that the latter are not covered by the treaty," he said. Gorbachev singled out France, Britain and NATO headquarters in Brussels, saying they "by no means intend to end the build-up of nuclear weapons. Quite the other way around."

He sharply criticized conservative western government officials, policy intellectuals and advisers.

"All sorts of 'analysts' and Kremlinologists" are trying to "frighten" and "intimidate" the public with "threats of 'catastrophic consequences' for the West if the disarmament process is carried on."

Gorbachev speculated that western skeptics and "rightists" who would "stop the disarmament train" come not only from the "military-industrial complex" and those "who live well by it," but also from those "who are afraid of a revival of the attractive force of socialist ideas."

He said, "They are scared because good feelings for our country are again growing, a new discovery of the Soviet Union is taking place. All that undermines the enemy image and hence the ideological fundamentals of anti-Soviet and imperialist policy. What served the reactionaries so well in the past decades is now falling to pieces."

Gorbachev said that arms control and other foreign policy initiatives are "now no longer an improvisation, not just a reaction to various other political moves and actions of the West, as it happened in the past."

Much of Gorbachev's speech was taken up with an ideological defense of reform. He was quick to counter charges from orthodox critics within the Soviet Union -- "defenders of socialism and mourners for Marxism-Leninism who believe both are under threat," as Gorbachev labeled them.

"There is confusion in the minds of some people," he said. "Are we not retreating from socialist positions, especially when we introduce new, unaccustomed forms of economic management and social life?" He was referring to new experiments with limited market principles such as cooperative businesses and decentralized economic planning.

Such reforms, Gorbachev insisted, are an attempt "to revive the Leninist look of the new system" and rid it of the "cult of personality" under Joseph Stalin, the "arbitrariness" under Nikita Khrushchev and the "stagnation" under Leonid Brezhnev. In his speeches since taking power in 1985, the 56-year-old leader has stressed the link between his own initiatives and the ideology of Vladimir Lenin, especially the relatively decentralized New Economic Policy of the 1920s.

Although Gorbachev lauded the socialist principle of guaranteed housing, jobs, education and medical care, he emphasized that the "extent of social protection in society depends on the amount of the national wealth."

He noted that while food production and housing starts have improved in recent years, the campaign against alcohol and the subsequent sharp drop in the production and sales of liquor have posed "a serious financial problem."