Soviet leader Mikhail Gorbachev told a U.S. Senate delegation today that the Soviet Union will make "radical proposals" to reduce strategic and intermediate-range offensive nuclear arms one day after the United States agrees to prohibit the militarization of space, according to Sen. Robert Byrd (D-W. Va.).

Gorbachev also told the bipartisan group of eight visiting senators, headed by Byrd, that the Soviet Union opposes research on military space defense programs such as the Reagan administration's Strategic Defense Initiative that goes beyond what is carried out in laboratories.

Clarifying the Soviet Union's objection to the Reagan administration's plans for research on the SDI antimissile defense system popularly known as Star Wars, Gorbachev said that any research outside of a laboratory would be considered "verifiable." Verifiable research and development on antimissile weapons, including those based in space, are subject to limits defined in the Antiballistic Missile Treaty ratified by both countries in 1972.

Gorbachev's remarks, and particularly his focus on SDI research, which is one of the most controversial issues between Moscow and Washington, are widely viewed in the western community here as a part of the Soviet positioning for the summit meeting between Gorbachev and Reagan set for Nov. 19-20 in Geneva.

Gorbachev projected an image of "pride," "toughness" and "alertness" during the 3 1/2-hour meeting in the Kremlin, the senators said at a press conference in Spaso House, the U.S. ambassador's residence here.

Although they were not swept away by his charm, Sen. John Warner (R-Va.) said, the senators found the Communist Party general secretary to be a dynamic figure.

"He seemed a man of some ability," Sen. Paul S. Sarbanes (D-Md.) said, "with a considerable degree of confidence and a lot of energy."

After the session with Gorbachev and the discussion on space weapons, Byrd said it "encourages us to believe that . . . he will be positive in his approach at the summit."

Byrd said that when he handed Gorbachev a letter from Reagan, the Soviet leader read it on the spot, smiled, "said it was a good letter" and relayed his return greetings to the president.

Sen. Sam Nunn (D-Ga.) cautioned, however, that "Gorbachev is a man who deals in substance. I think it would a serious mistake for the president to go into the summit thinking it will all be resolved by warm personalities."

In promising unspecified "radical" proposals for arms reductions contingent on a resolution of the disagreements over SDI, Sen. George Mitchell (D-Maine) said, the Soviets are clearly offering an "inducement" for movement on SDI.

Nunn, an arms control specialist, said Gorbachev's definition of research was not a "substantive breakthrough" but "a step in the right direction" because it represented the first time the Soviet Union has specified what it means in objecting to SDI research.

But Nunn objected to Gorbachev's definition -- limiting research to laboratories -- calling it "too narrow." It would probably be "unacceptable" to the White House, he said, because it is more limited than what the administration's SDI plans call for and more limited than what the Soviets have already done in the field of space-based defense.

Furthermore, Nunn said, now that Gorbachev has clarified the limits of research acceptable to him, the Soviet negotiators in Geneva should formally propose it as the research limit acceptable to them for the U.S. Star Wars program so that the United States can respond. Until now the Soviet team at Geneva has not done that, he said.

Warner said he was also "disappointed" that Gorbachev failed to admit to the levels of antisatellite testing and space defense research that the Soviet Union has pursued.

Nunn added that in making its would-be proposals on offensive weapons contingent on an agreement on SDI, the Soviet Union is putting forward a condition not consistent with the January agreement between Secretary of State George P. Shultz and then-Soviet foreign minister Andrei Gromyko, who has since become Soviet president.

The two agreed then that the talks in Geneva would be conducted simultaneously on intermediate weapons, strategic weapons, and space weapons.

Gorbachev, focusing his remarks on arms control, also argued that a long-term moratorium on nuclear testing effectively would make existing weapons systems obsolete.

But he brushed lightly over the subjects, raised by the senators, of the Soviet involvement in Afghanistan and Soviet human rights abuses in a manner that one described as "unfortunate." In the discussion on SDI research, Gorbachev, according to Warner's notes on the meeting, said, "You can't verify what's going on in the brain . . . and that's what we refer to as fundamental or basic research."

"But as soon as you go beyond the laboratory, go to mock-ups, models, contracts with defense contractors," Warner quoted Gorbachev as saying, "here surely verification can be done."

Gorbachev continued, "We want a ban on that phase of research that approaches design and manufacture," Warner said.

"The challenge" Nunn said, "is to table that in Geneva."

Several of the senators portrayed Gorbachev as a formidable summit partner for Reagan.

"Being proud," Warner said, "he'll go into that summit with the perception that no one will come out a winner and no one a loser. I got the clear impression that it's going to be give and take. He's going to give, and he's going to take, as long as he's accepted as an equal."