President Reagan today made public new U.S. proposals to reduce the number of medium-range nuclear weapons in Europe, and challenged the Soviet Union to "match our flexibility."

"The door to an agreement is open," Reagan said. "It is time for the Soviet Union to walk through it."

In a major speech about East-West relations on the first day of debate of the 38th session of the U.N. General Assembly, Reagan sought to take the initiative in the deadlocked Intermediate Nuclear Forces (INF) negotiations in Geneva by offering the Soviets an advantage in European-based medium-range nuclear weapons in exchange for equal global limits on such warheads.

He also sought to capture the moral high ground of international debate, referring to the recent shooting down of a South Korean airliner as "a timely reminder of just how different the Soviets' concept of truth and international cooperation is from the rest of the world."

"Peace cannot be served by pseudo-arms control," Reagan said. "We need reliable reciprocal reductions. I call upon the Soviet Union today to reduce the tensions it has heaped on the world in the past few weeks, and to show a firm commitment to peace by coming to the bargaining table with a new understanding of its obligations.

"I urge it to match our flexibility. If the Soviets sit down at the bargaining table seeking genuine arms reductions, there will be arms reductions."

Despite Reagan's blunt challenge to the Soviets, his speech today was far more muted than his June 17, 1982, address to the U.N. disarmament conference, when he accused the Soviets of atrocities and "ruthless repression."

Today's speech also was received more enthusiastically by many U.N. delegates, who said afterward that they welcomed Reagan's support for the United Nations, "the ideals of the U.N. Charter" and the principles of arms control and nonalignment.

U.N. Secretary-General Javier Perez de Cuellar said of Reagan's presentation on nuclear arms: "It is a very important proposal. It deserves careful attention. It is a positive statement that . . . emphasized the necessity of maintaining the disarmament dialogue and providing specific proposals."

But the Soviet delegation did not join in the applause following Reagan's speech. The most prominent of the Soviets' six seats in the General Assembly was left vacant, according to Soviet Ambassador Oleg Troyanovsky, as a reminder of the absence of Soviet Foreign Minister Andrei A. Gromyko.

Troyanovsky would not comment on Reagan's new nuclear arms control proposal. But another Soviet delegate, Richard Ovinikov, characterized the proposal as "a sugar-coated ploy to deploy U.S. missiles."

He was referring to scheduled U.S. deployment, beginning in December, of new Pershing II and ground-launched cruise missiles with nuclear warheads in several NATO alliance countries in western Europe.

The Soviets announced later yesterday that their speech to the General Assembly, normally given on the second day of the session, was being postponed from Tuesday until Oct. 4 for "technical reasons" they did not explain.

In his speech, Reagan called on the Soviet Union "to show proof that it wants arms control in reality, not just in rhetoric."

The president said he had instructed Paul H. Nitze, the chief U.S. negotiator at the INF talks in Geneva, to put forward a package of new proposals after extensive consultations with the NATO allies and Japan.

In addition to expressing a willingness to deploy fewer U.S. Pershing II and cruise missiles in Europe in exchange for a Soviet agreement on equal global limits of medium-range missile warheads, Reagan said "the United States will consider mutually acceptable ways to address the Soviet desire that an agreement should limit aircraft as well as missiles."

U.S. officials emphasized that the deployment of the new NATO missiles will continue on schedule unless an agreement is reached with the Soviets. By U.S. accounting, the Soviets already have deployed 351 SS20 missiles, 243 of them targeted on western Europe. Each SS20 is armed with three nuclear warheads.

Secretary of State George P. Shultz said after the president's speech that the United States had received no indication that the Soviets were receptive to the new U.S. proposal, which Nitze already has submitted to Soviet negotiatiors in Geneva. Shultz said the Soviets "say they are interested in a reasonable settlement and our proposals are reasonable . . . and show opportunities for give-and-take."

Reagan also reemphasized his commitment to reducing U.S. and Soviet arsenals of long-range nuclear missiles, the subject of separate negotiations in Geneva. During the last round of those Stategic Arms Reduction Talks (START), the United States made new proposals that American officials said were designed to deal with Soviet objections.

"We will continue to build upon this initiative," Reagan promised.

At another point, the president said, "A nuclear war cannot be won and must never be fought."

Reagan also sought to reassure nonaligned nations that they did not have side with the United States in disputes between the superpowers.

Last week, a senior administration official in Washington complained that a "double standard" favoring the Soviets prevailed too often at U.N. meetings. Reagan tried to make the same point in a more diplomatic way, reminding delegates that the "nonaligned movement was founded to counter the development of blocs and to promote detente between them.

"Its founders spoke of the right of smaller countries not to become involved in others' disagreements. Since then, membership in the nonaligned movement has grown dramatically, but not all the new members have shared the founders' commitment to genuine nonalignment.

"Instead, client governments of the Soviet Union, who have long since lost their independence, have flocked into the nonaligned movement, and once inside have worked against its true purpose. Pseudo-nonalignment is no better than pseudo-arms control."

In a subsequent passage aimed at dividing the Soviet Union from nonaligned countries supporting it, Reagan said the United States "rejects as false and misleading the view of the world as divided between the empires of the East and West."

"The United States does not head any bloc of subservient nations, nor do we desire to," Reagan said. "What is called the West is a free alliance of governments . . . . What is called the East is an empire directed from the center which is Moscow."

Perez de Cuellar said he was pleased with Reagan's positive comments about the United Nations after recent exchanges between the U.S. and Soviet delegations over keeping the U.N. headquarters in this country.

Last week Reagan said many Americans agreed with Deputy U.S. Ambassador Charles M. Lichenstein, who said the United Nations could move out of the United States if it wanted to.

By contrast, Sen. Daniel Patrick Moynihan (D-N.Y.), the U.S. representative to the United Nations during the Nixon administration, said he felt the president's words today reaffirmed U.S. commitment to the international organization. "Clearly, the U.N. should stay in New York," Moynihan said.

A Senate vote last week that would reduce U.S. contributions to the world body "signaled a sense of the Congress that we don't get enough support there," Moynihan said. Reagan's "speech should help to rebuild our bridge to the United Nations."

Reagan also held separate private meetings during the day with Moroccan King Hassan and Indian Prime Minister Indira Gandhi.

Aboard Air Force One returning to Washington this afternoon, White House spokesman Larry Speakes said Reagan was gratified with the "very positive" response to his speech.

"He was satisfied that his speech conveyed the message he wanted to convey," Speakes said. "He was pleased with the content as much as with any speech he's delivered."