Soviet President Leonid Brezhnev announced sweeping arms control initiatives today, while warning that Moscow would take "retaliatory steps" if the United States proceeds with its missile deployment plans in Western Europe.

The introduction in Western Europe of Pershing II and cruise missiles "capable of striking targets on the territory of the Soviet Union," Brezhnev said, would compel the Soviets to put the United States in an "analogous position." Well-informed Soviet sources said this was intended to suggest the possibility of introducing nuclear weapons in Cuba.

Brezhnev said that Moscow had unilaterally halted deployment of its SS20 medium-range nuclear rockets in the European part of the Soviet Union.

The Soviet leader voiced the hope that a "sensible agreement" could be reached at the Soviet-American talks in Geneva on reducing medium-range nuclear weapons in Europe. He said the issue was "the key to ending the growing danger of a world-wide nuclear missile war."

But, in a speech before more than 5,000 cheering persons in the Kremlin's Palace of Congresses, Brezhnev asserted that "if the governments of the United States and its NATO allies . . . were actually to carry out their plan to deploy in Europe hundreds of new American missiles capable of striking targets on the territory of the Soviet Union, a different strategic situation would arise in the world.

"There would arise a real additional threat to our country and its allies from the United States," he said. "This would compel us to take retaliatory steps that would put the other side, including the United States itself, its own territory, in an analogous position. This should not be forgotten."

Western military specialists, who described the tone of Brezhnev's speech as "ominous," also said he most likely had Cuba in mind when he spoke about placing the United States in a position "analogous" to the one faced by the Soviet Union following the scheduled deployment next year of the new U.S. nuclear missiles in Western Europe.

While the Soviet Union already has intercontinental missiles capable of reaching the United States, the placement of shorter range missiles in Cuba would mean the United States would have less warning time after launch and that the missiles would be coming from a direction inadequately guarded by an electronic detection screen.

In 1962, Soviet leader Nikita Khrushchev attempted to install Soviet nuclear missiles in Cuba, setting off a major confrontation between the United States and the Soviet Union. Military specialists here pointed out, however, that the Soviets have since developed medium-range nuclear aircraft and that conceivably these could be introduced into Cuba with a far greater ease and speed.

U.S. sources interpreted Brezhnev's statement as being less threatening than the military specialists read it to be, noting that while his threat seemed "explicit," it was vaguely formulated to suggest "mostly bluster," presumably to influence or scare the U.S. public. They also pointed out that the Soviets have repeatedly reaffirmed the validity of a 1962 understanding between Moscow and Washington that ended the Cuban crisis.

Under the understanding, according to U.S. officials familiar with the negotiations, the Soviets pledged not to reintroduce offensive missiles into Cuba.

Brezhnev, in his first major foreign policy address since the Communist Party congress a year ago, said the question of new U.S. nuclear missiles in Western Europe "is yet another reminder" about the importance of Soviet-American strategic nuclear arms limitation and reduction talks. Those negotiations, known as the SALT talks, have been going on for more than a decade and deal with intercontinental nuclear weaponry. They are separate from the current Geneva talks, which are devoted to European-based nuclear arms and which were recessed today until May.

"We call on the government of the United States not to raise artifical barriers to SALT talks and to get down to them in the nearest future," he said.

"Pending their resumption, we would propose that the two sides undertake a mutual commitment not to open a new channel of the arms buildup, not to deploy sea-based or ground-based, long-range cruise missiles."

Calling for "maximum restraint in the military activity" of both sides, Brezhnev added a new item to his standard list of arms control proposals.

The Soviet government, he said, would be prepared to negotiate a "mutual restriction of naval operations."

"In particular," he continued, "we would consider it possible to agree that missile submarines of the two sides should be removed from their present extensive combat patrol areas, that their cruises should be restricted by limits mutually agreed upon.

"We would also be prepared to discuss the question of spreading confidence-building measures to the seas and oceans, especially to areas through which the busiest shipping routes pass."

Brezhnev's harsher tone and the warning hinting at the possible introduction of Soviet nuclear arms nearer America's shores were interpreted by Western analysts as a slight yet important shift of policy.

There have been indications here that the Kremlin political leadership was under pressure from its military chiefs to come up with a more assertive response to President Reagan's quest for military superiority over the Soviet Union.

Marshal Nikolai Ogarkov, the Soviet chief of staff, recently expressed the view that the United States was using the Geneva talks as a smokescreen to that purpose.

From Moscow's point of view, the planned deployment of the 572 U.S. medium-range rockets in Europe would almost immediately tilt the strategic balance of forces in America's favor.

The NATO decision to deploy the American missiles was based on the perceived need in the West to counter Moscow's buildup of its SS20 missiles, each carrying three warheads. The missiles are mobile and able to hit targets in Western Europe but not the United States.

The Soviets have contended that they were merely carrying out modernization of their medium-range forces, replacing the old SS4 and SS5 missiles with the new SS20s.

Largely under the pressure from West Europeans, the United States agreed to enter the Geneva talks Nov. 30 aimed at reducing or eliminating these missiles. On the American side, the starting point was Reagan's "zero option," under which NATO would abandon plans to deploy the 572 new U.S. missiles in Western Europe if the Soviets dismantle their SS20, SS4 and SS5 missiles.

The Soviets have rejected Reagan's plan. They expressed readiness to negotiate sharp reductions of all medium-range nuclear weapons in Europe including U.S. forward-based systems and the independent French and British nuclear forces. These would include air and seaborne systems.

During a visit to Bonn in November, Brezhnev expressed readiness to reduce "unilaterally" a certain part of these weapons and offered a moratorium on the stationing and production of medium-range weapons while the Geneva talks were in progress.

Today Brezhnev went a step further saying, "The Soviet leadership has taken a decision to introduce, unilaterally, a moratorium on the deployment of medium-range nuclear armaments in the European part of the U.S.S.R.

"We are freezing, in both the quantitative and qualitative respects, the armaments of this kind already stationed here and are suspending the replacement of old missiles, known as SS4 and SS5, by newer SS20 missiles.

"The Soviet Union intends already this year, unless there is a new aggravation of the international situation, to reduce a certain number of its medium-range missiles on its own initiative . . . "

He described these actions as a gesture of good will. He said the moratorium "will be in force either until an agreement is reached with the United States to reduce on the basis of parity and equal security the medium-range nuclear weapons designed for use in Europe or until that time" when the United States takes "practical preparations to deploy" Pershing II and cruise missiles.

The Soviet leader at the same time asserted that Reagan's "militarist line and aggressive policy" are "forcing us to maintain our country's defense capability at the due level.

"That is a grim necessity of the present-day world and, of course, it requires diverting considerable resources to the detriment of our plans for peaceful construction."

During a one-hour speech at the opening session of the Soviet trade union congress, Brezhnev also made it clear that he expected the United States to observe the provisions of the SALT II agreement. The U.S. Senate never ratified the pact and it expired at the end of 1981.

Deployment of the cruise missile, which has a range of more than 380 miles, was banned under a protocol to the SALT treaty. Brezhnev urged that both sides refrain from the depoyment of the long-range cruise missiles, and the Soviets seem to be concerned about U.S. plans to deploy them at sea.

It is believed that the United States is well ahead of the Soviet Union in this area of technology.

Brezhnev's call for a resumption of strategic arms limitation talks was coupled by sharp criticism of the Reagan administration. He described its policies as "dangerous balancing on the brink of a real war" and assailed its "unmatched cynicism" and the use of the threat of force.

"It is simply astonishing to see it all," Brezhnev said. "You cannot help asking yourself what is there more of in this policy--thoughtlessness and lack of experience in international affairs or irresponsibility and, to say it bluntly, an adventurist approach to problems crucial for the destiny of mankind."