The official Soviet press today sharply criticized President Reagan's plan to switch U.S. strategic posture to a nuclear deterrent based on an antiballistic missile defense, saying it would be a major American step toward a "first-strike" nuclear capability against the Soviet Union and a violation of the 1972 ABM treaty.

The Soviet reaction to Reagan's speech of last night came on the same day in which it was announced here that veteran Soviet Foreign Minister Andrei Gromyko has been named a first deputy premier in an unexpected move apparently calculated to strengthen the executive branch of the Soviet state.

By adding an internationally known figure who also is respected here, the new Soviet leadership may have sought to improve its image at home and abroad.

Political observers here speculated that the appointment of Gromyko to a top government position reflects a decision to concentrate all foreign policy matters in his hands. Such a move was needed, according to this view, because the Soviets are believed to be preparing for what they expect will be a major showdown with the United States.

In the first Soviet reaction to Reagan's television address, the authoritative government news agency Tass swiftly charged that "the deployment of such antimissile defense systems would be a direct violation of Soviet-American treaties on ABM and the protocol regarding them," Tass said.

Later today the official press agency Novosti said Reagan's "directive clearly indicates his intentions to perpetuate the arms race and to carry it over to the next century." It said, "This is another step by his government to undermine everything positive that has been achieved in Soviet-American arms control by the previous U.S. administrations."

Reagan's speech, Tass said, raised concerns in Moscow about "new policies of the United States aimed at achieving superiority in nuclear armaments over the Soviet Union and destroying the approximate balance of power existing in the world."

Tass quoted U.S. officials as having asserted that Reagan's plan envisages the establishment of antiballistic missile systems based on Earth and in orbit and added that the United States "is already spending $1 billion" annually on the development of such new weapons.

Tass stressed that the treaty gives the United States the right to deploy antiballistic missiles only at one site. It also contended that Reagan's decision to mount a long-term research and development program on antiballistic systems runs counter to the 1972 pact.

U.S. officials have stressed that the ABM treaty expressly permits spending for research and development.

Reagan said his proposal was consistent with the treaty. He argued that a shift toward the ABM-based deterrent would mean that by the end of the century the United States would abandon the policy of deterring Soviet attack with a threat of instant nuclear retaliation in favor of defense systems capable of destroying Soviet missiles before they could reach their targets.

Tass characterized his address as another instance of "military hysteria" designed to push through Congress the administration's "mammoth arms buildup programs."

The one-sentence announcement about Gromyko, distributed by Tass, suggested that the 73-year-old Soviet figure would also retain the job of foreign minister that he has held for more than a quarter of a century.

The appointment appeared to further consolidate Gromyko's influence in the new Kremlin leadership of Communist Party chief Yuri Andropov, who reportedly is convalescing at his country home from a kidney ailment.

Two others also hold the title of first deputy to Premier Nikolai Tikhonov. Gaidar Aliyev, 59, a career officer of the Soviet secret police KGB, was named Nov. 24, only two days after being promoted to full Politburo membership. The third man, Ivan Arkhipov, 75, is a technocrat who does not belong to the top leadership.

Although he has been the personification of Soviet diplomacy for almost four decades, little is known about Gromyko's personal life. Even in this country, only a handful of people know that he is a trained economist who holds not only a doctorate in economics but also a doctorate of science in political economy--the highest academic degree here.

In his new role as first deputy premier and foreign minister, Gromyko is expected to supervise not only Moscow's diplomacy but also its foreign trade, scientific and technological exchanges and other aspects of foreign relations currently handled by various ministries.

There was speculation among western diplomats here that Gromyko may eventually relinquish his Foreign Ministry position.

Gromyko's appointment was announced following a meeting today of the Politburo.

A communique issued after the Politburo meeting clearly indicated that the ruling body had not held its regular meeting last week, presumably because of Andropov's illness. It was not clear, however, whether Andropov attended today's session.

The Associated Press quoted well-informed Soviet sources as saying that Andropov attended the meeting.

It had long been believed that Gromyko would not rise within the Soviet hierarchy because he lacked a power base.

But at a time when the leadership was in a state of flux, his influence became greater. He provided the experience and steadying influence of a man who conducted Soviet foreign policy for 26 years and who possessed a penetrating knowledge of the world and its leaders that no other foreign minister could match.

He was Stalin's ambassador to Washington during World War II, attended the Tehran, Yalta and Potsdam conferences and later represented Moscow at the United Nations. He was named foreign minister by Nikita Khrushchev in 1957.