Gorbachev Replaced : Vice President Takes Over, Tass Says : Soviet Union: Health reasons are cited. A state of emergency is put into effect in some areas by a new ruling committee of hard-liners.

MOSCOW —  Mikhail S. Gorbachev, whose bold reform policies brought democracy to the Soviet Union and ended the Cold War, was replaced early today as president of his country, ostensibly for health reasons.

Vice President Gennady I. Yanayev, named the chairman of the “State Committee on the Emergency Situation in the U.S.S.R.,” took over as president. A six-month state of emergency in “individual localities” was put into effect “with the aim of overcoming the profound and comprehensive crisis, political, ethnic and civil strife, chaos and anarchy that threaten the lives and security of the Soviet Union’s citizens and its sovereignty, territorial integrity, freedom and independence,” the committee said.

“A mortal danger has come to loom large,” declared the committee, whose membership is dominated by known hard-liners. The reform campaign by Gorbachev--which caught the attention of the world but virtually destroyed the basis of the socialist economy without building anything in its place, leaving many of his own people hungering for a materially better life--”has gone into a blind alley,” the committee said.

The committee’s declaration, issued in Yanayev’s name, was read repeatedly over the radio and television in a monotone voice and issued by the official Tass news agency.

Tass gave no details about the 60-year-old Gorbachev’s illness, nor did it say whether the appointment of Yanayev, 53, is permanent.

The emergency measures “in no way indicate a refusal from the course toward deep reforms in all spheres of life, state and society,” Tass said. “These measures are necessary and are dictated by the vital need to save the economy from collapse.”

But despite such assurances, it was clear that only the most arch-conservative members of the Kremlin leadership, including Interior Minister Boris K. Pugo and Defense Minister Dmitri T. Yazov, KGB Chairman Vladimir Kryuchkov and Prime Minister Valentin S. Pavlov, were announced as members of the committee. And the new panel warned that it would not brook any interference by foreign countries in Gorbachev’s support.

“We want to live with everyone in peace and friendship. But we firmly declare that no one will ever be allowed to infringe on our sovereignty, independence and territorial integrity,” the committee proclaimed.

Gorbachev has been in the Crimea on vacation. He looked wan and drawn in his last television appearance, a speech to the nation in early August, but there was no way to judge whether he was, in fact, ailing, and no information on where he was this morning.

Only Friday, Alexander N. Yakovlev, who long had been Gorbachev’s alter ego, had resigned under pressure from the Soviet Communist Party, warning that Stalinists were plotting a government and party coup.

Moscow’s main television center was surrounded by soldiers with automatic weapons this morning. And a half-dozen armored personnel carriers were seen clanking through western Moscow not far from the U.S. Embassy. Echo Moscow, a liberal radio station, reported that tanks were approaching the capital from the southeast.

There was no immediate reaction from the White House. Spokesman Doug Davidson, in Kennebunkport, Me., where President Bush is vacationing, said he had just heard news reports of the development and was checking with other officials.

In response to the Moscow news, the dollar jumped against other currencies, and share prices sank in Tokyo and Hong Kong markets today. Share prices on the Tokyo Stock Exchange plummeted over 1,000 points.

Gorbachev, who has been Soviet leader since March, 1985, was to have been in the capital on Tuesday to officiate at the first signing of his Union Treaty, which was his solution to keeping the country together by allocating more powers to the individual republics.

He had left Moscow on Aug. 4 on summer vacation, four days after he and Bush signed the historic Strategic Arms Reduction Treaty on cutting back nuclear weapons amid Kremlin fanfare.

The circumstances of his replacement immediately recalled the successful plot mounted in the Communist Party Central Committee against Nikita S. Khrushchev in 1964, which began when he was on vacation in the Soviet south.

The last straw for the right-wingers appeared to be Tuesday’s scheduled signing of the Union Treaty, which would have mandated a historic decentralization of political power. The Cabinet of Ministers, led by Pavlov, criticized the treaty over the weekend, and Anatoly I. Lukyanov, the chairman of the Soviet legislature and a known conservative, vigorously denounced the treaty early today in a statement issued by Tass, calling it insufficient to protect a unified national economy and state property.

Yanayev, seemingly a bland career Communist functionary, was plucked from obscurity by Gorbachev last December to be his vice president. He had headed the now discredited official trade union movement, but before that had been the leader of the Komsomol Young Communist Youth League for several years, starting at the age of 31.

Boris N. Yeltsin, the radical president of Russia, was said by acquaintances to be huddled at his country estate outside Moscow with Russian Federation government leaders. He has long butted heads with Gorbachev, but the two men had established a modus vivendi after agreeing on the Union Treaty.

The committee served notice on Yeltsin and the other leaders of the often rebellious republics that beginning today, they would have to heed Moscow’s will without question. “The constitution and laws of the U.S.S.R. have unconditional priority throughout the territory of the U.S.S.R.,” Tass said, in summarizing the committee’s statement.

It was Gorbachev’s selection in March, 1985, to succeed Konstantin U. Chernenko as general secretary of the Communist Party that brought him to the supreme position of power in the Kremlin.