Soviet President Leonid Brezhnev today firmly rejected President Reagan's offer of a "zero solution" for land-based, medium-range nuclear weapons in Europe, calling it an attempt to "disarm us." Instead he proposed his own plan for nuclear arms reduction in Europe.

Renewing an offer for a U.S.-Soviet moratorium on the deployment of new medium-range nuclear weapons in Europe, the Kremlin chief, speaking at a dinner on the first full day of his official visit to West Germany, also said Moscow was prepared to reduce unilaterally "a certain portion" of its medium-range rockets in the European part of the Soviet Union.

"As a gesture of good will," Brezhnev said, "we could reduce unilaterally a certain portion of our medium-range nuclear weapons in the European part of the Soviet Union. We could reduce, so to say, as an advance payment in the expectation of moving toward a lower level about which the Soviet Union and the United States could reach an agreement in the course of the negotiations" that open next week in Geneva.

"This," Brezhnev declared, "is a new and essential element in our position."

He coupled his peace gesture with an attack on the United States, accusing it of launching a "powerful new program" of rearmament that includes the production of neutron weapons and designs for a limited nuclear war on Europe.

Although Brezhnev described his offer for a unilateral arms reduction as new, the plan did not appear to go beyond a proposal he made in Berlin in 1979 just before the North Atlantic Treaty Organization decision to deploy the 572 U.S. missiles beginning in late 1983. That decision also contained an offer to negotiate with the Soviet Union on curbing such weapons.

Brezhnev's remarks indicated that Moscow and Washington would be going into the Geneva negotiations with broadly diverging positions.

Commenting on Brezhnev's speech, West German Chancellor Helmut Schmidt said in a television interview that it contained a "somewhat different combination" of old ideas but did not dismiss it.

"What seems to me worth singling out," Schmidt said, "is that the Soviet Union has made clear, beyond any doubt, that it is ready to reduce its medium-range weapons in Europe. It has linked this with terms and conditions which must be examined more closely."

In Washington, a White House spokesman said there would be no immediate comment on Brezhnev's proposal.

Bonn government spokesman Kurt Becker told reporters that in several hours of closed-door discussions today Schmidt had pressed the Western view that Soviet medium-range nuclear missiles pose an intolerable threat to Western Europe.

Becker quoted Schmidt as urging Brezhnev to consider Reagan's offer to cancel the planned U.S. deployment if the Soviets would "remove" their new SS20 and old SS4 and SS5 nuclear missiles targeted against Western Europe and China.

Schmidt's use of the word "remove" to explain the zero solution differed from Reagan's choice of "dismantle" in his speech setting forth the offer last week and may have been a subtle attempt on Schmidt's part to signal some bargaining flexibility on a very complex and sensitive issue.

The official Western position is that nothing short of destruction of the Soviet land-based medium-range system would be satisfactory, since the Soviet weapons are mobile and could be rolled back if moved to the Asian part of the Soviet Union.

But the Soviets are expected to fiercely resist scrapping any of the more than 250 multiple warhead SS20s, an expensive new weapons system. According to Becker, though, Schmidt also made clear to Brezhnev that the aim of the Geneva talks would be reductions in such systems. Schmidt was said to have called unacceptable any arrangement that would simply put the Soviet weapons behind the Ural Mountains.

Nonetheless, Moscow in recent months has been able to go over the heads of Western governments to use its peace offers to stir grass-roots opposition in Western Europe to the new U.S. weapons. Brezhnev appeared determined not to retreat from a so-far successful propaganda campaign.

"We consider the situation alarming," Brezhnev said in his speech. "The West's largest power is attempting to escalate the spiral of armaments."

Brezhnev said the U.S. Pershing II and cruise missiles planned for Europe presented "an unprecedented threat to the continent" and added that the deployment plans together with Reagan's recent arms control proposals caused him to doubt America's interest in serious negotiations.

The Soviet Union "would never accept" the Reagan proposal, Brezhnev declared, saying it would upset what he claimed is an "approximately equal number of Western and Soviet nuclear weapons already deployed in Europe."

The Western governments dispute this claim of existing parity, asserting that Soviet medium-range nuclear weapons far outnumber those belonging to the United States, Britain and France.

Brezhnev, who will be 75 next month and is known to have had many health problems, seemed to be standing up well under the strains of the first of two days of intensive talks with Schmidt and other senior West German officials.

West German sources close to today's discussions described his performance as forceful and, one said, "quite vivid."

Despite Brezhnev's strongly worded public message, spokesmen for both men said the talks were taking place in a "very good atmosphere" and in an "atmosphere of trust." The two leaders, meeting for the sixth time in seven years, reportedly covered a wide range of issues from Afghanistan to Central America. But their discussions were clearly dominated by the weapons issue.

In a briefing, a Soviet spokesman, Leonid Zamyatin, decried what he described as sudden Western concerns about Soviet rockets that had started to appear as early as 1975. Zamyatin quoted Brezhnev as pointing out the "exceptionally important fact" to Schmidt that while Soviet medium-range rockets cannot reach U.S. soil, the American systems can reach Soviet soil.

Becker, in turn, told reporters Schmidt voiced understanding for Soviet fears of Western nuclear systems in Europe. "But similarly," Schmidt reportedly told Brezhnev, "we also see your rockets as a threat to us. I have never thought you would ever press the button, but just the existence of these weapons has grave effects."

Thus, Schmidt said, he favors the zero solution, adding that he hoped it could be agreed to in "the first stage" of the Geneva talks.

In his own dinner remarks as well as in their private sessions, Schmidt reminded Brezhnev that during the Soviet president's last visit here in 1978, they signed a communique that affirmed the principles of approximate equality and parity in nuclear systems.

But since then, Schmidt noted, "new developments have occurred which have disturbed us greatly." He cited deployment of Soviet SS20s.

Schmidt warned Brezhnev that if no agreement on reductions in land-based medium-range nuclear systems in Europe is reached by mid-1983, West Germany would honor its commitment to accept some of the new U.S. weapons on its soil.

Also contributing to this report was Washington Post Moscow correspondent Dusko Doder who is also covering the meeting in Bonn.