President Reagan said yesterday he "welcomed" word that the Soviet Union will propose a 50 percent cut in strategic offensive arms in return for strategic defensive restrictions at Geneva this week and declared, "We are ready for tough but fair negotiating."

Reagan, in his weekly radio address that was notable for its moderate language about the Soviet Union, did not divulge the particulars of the proposal, which was presented to him in general terms at the White House Friday by Soviet Foreign Minister Eduard Shevardnadze.

Nevertheless, Reagan's comments about the Soviet "counterproposal" set forth several U.S. concerns reported to be under intensive discussion by the administration. "It's important that the counterproposal address our concerns about reductions and stability, just as we have sought to address Soviet concerns," Reagan said.

His statement appeared to reflect U.S. uncertainty about whether, and to what extent, the Soviet proposal would encompass reductions in the large land-based intercontinental missiles that are the core of Moscow's nuclear force and which the United States considers most "destabilizing" because of their capacity to strike U.S. land-based missile sites.

U.S. experts said Moscow's 50 percent reduction would be from strategic nuclear "charges" that include missile warheads, nuclear bombs and short-range attack missiles aboard bombers and other weapons capable of striking Soviet or U.S. territory. Without further "sublimits" on particular weapons, such a proposal would do little to cut back Moscow's heavy land-based missiles.

Reagan also said, "We hope it will be free from preconditions and other obstacles to progress." This seemed to refer to a central Soviet condition reportedly attached to its cutback plan: the cessation of all work on what the Soviets call "space strike arms." This would effectively ban Reagan's cherished Strategic Defense Initiative (SDI), or "Star Wars," program.

In his radio address, Reagan once again spoke up for SDI, although he was careful to describe it as "research." During the meeting with Shevardnadze, the president reported, "I emphasized the need for a more productive Soviet response to our offer at Geneva to begin the U.S.-Soviet dialogue now on how to fashion a more stable future for all humanity if the research on strategic defensive technology, which both the U.S. and U.S.S.R. are conducting, bears fruit."

U.S. experts interpreted Shervardnadze's call for a cessation of all work on space strike arms as a call for a ban on research as well as testing, development and deployment of SDI, although the Soviet foreign minister reportedly never mentioned the word "research."

On some occasions, various Soviet officials have seemed to concede that basic research could not be banned. In his interview with Time magazine, Soviet leader Mikhail Gorbachev drew a distinction between "fundamental research" on SDI and activity that clearly should be banned.

Among the key questions to be explored about the Soviet proposal, therefore, is whether the Soviets would agree to continuation of "research" on SDI if the "testing" and "development" stages were to be further limited, and what the real effect of any such limits would be.

Former President Jimmy Carter, at ceremonies in Lewiston, Maine, honoring ex-secretary of state Edmund S. Muskie, said he saw little hope for agreement unless Reagan "is willing to bend" on SDI. Carter called the "Star Wars" plan "ill-conceived, expensive and an impediment to arms control."

Rep. Nick J. Rahall II (W.Va), presenting the Democratic response to Reagan's address, said Democrats "are glad you Reagan have seen the light and are going to meet with Mr. Gorbachev in a summit. Our hope is that through this meeting we can prevent a militarization of the heavens."

Soviet negotiators are expected to provide details of Moscow's new proposal in Geneva in two special meetings Monday and Tuesday. U.S. sources said the Soviets had asked on Sept. 19 that the meetings be set up, giving Washington officialdom a tipoff that long-promised new proposals were on the way.

Gorbachev is to arrive in Paris Wednesday on a three-day visit, his first trip to the West since becoming Soviet leader last March. There was speculation in Washington that the new Soviet arms proposal might be made public by Gorbachev during his visit to the French capital.

Shevardnadze, who has been in this country two weeks to attend the United Nations General Assembly and to meet Reagan and Secretary of State George P. Shultz, flew to Moscow yesterday to join Gorbachev for the visit to Paris.

Reagan, in his radio address, said he told Shevardnadze Friday that "I am hopeful" about his Nov. 19-20 summit meeting with Gorbachev and that "I put forward some new ideas as well as my plans and expectations for that meeting." A White House official said Reagan appeared to be referring to "his personal approach to resolving the problems" rather than any new U.S. proposals.

In contrast to some of the harsh words from Reagan about the Soviet Union in the past, the president yesterday spoke of "our efforts to build a more constructive and stable long-term relationship with the Soviet Union."

Reagan went on to describe U.S.-Soviet differences as "fundamental in political systems, values and ideologies as well as in the way we conduct our relations with other countries." He said problems between the two nations should be solved where possible and, where not possible, the two powers should "manage our differences in a way that protects western freedom and preserves the peace." In an attempt to exchange viewpoints in a region of serious U.S.-Soviet differences, Shultz and Shevardnadze agreed late Friday to authorize special discussions by their aides next month in Washington on Latin American issues, especially Central America and the Caribbean, according to sources. Assistant Secretary of State for American Republic Affairs Elliot Abrams is expected to lead the U.S. delegation.

Reagan also said he and Shevardnadze "agreed to set up a series of senior-level discussions between our experts in preparation" for the summit meeting.

No schedule of such advance consultations has been agreed on, the White House official said. Among the possibilities being discussed are another visit by Shervardnadze to New York for the United Nations 40th anniversary celebration next month, a trip to Moscow by Shultz or a Moscow trip by White House national security affairs adviser Robert C. McFarlane.