Soviet leader Mikhail Gorbachev today zeroed in on the U.S. Strategic Defense Initiative as the overriding issue of next week's summit in Geneva.

In what may be his last public speech before he meets President Reagan in Geneva Tuesday, the Soviet leader said that U.S. pursuit of the space-based defense program would escalate East-West tensions to "unprecedented" levels.

"Whether strike weapons will make it to outer space or be barred from it is an all-important question," Gorbachev said. "The answer to it is decisive to the course of developments in the world for many years ahead."

Speaking before a group of Nobel Prize winners at the Kremlin, Gorbachev pledged an "open and fair" approach to the Geneva meeting. "We go there for serious and productive work and, I should say, with our hands not empty."

Gorbachev's speech, aired tonight on national television, also made a pitch to the broader domestic audience as the Geneva meeting nears.

Although staking out a hard line on SDI, Gorbachev generally struck a neutral tone, emphasizing hopes for U.S.-Soviet cooperation and avoiding biting rhetoric against the United States or the Reagan administration.

Gorbachev praised such joint science projects as the Soyuz-Apollo space flight and the thermonuclear reactor known as Tokamak.

The positive note was in contrast to recent commentary in the Soviet press that has attacked the administration on several fronts, including its recent proposal at the arms control negotiations in Geneva.

Today's issue of the Communist Party daily Pravda, for instance, carried articles accusing the United States of teaming up with Israel for "sinister" military exercises and of viewing Western Europe as "a battlefield" for nuclear war.

In Gorki today, Soviet President Andrei Gromyko also took a hard line, accusing "American imperialism" of waging "flagrant psychological warfare" against the Warsaw Pact and trying to "dupe" people with the SDI program.

The meeting in Geneva next week will be the first high-level U.S.-Soviet encounter in 40 years to take place without Gromyko present.

In his speech today, the 54-year-old Gorbachev used an approach that differed from the emphatic statements favored by Gromyko. Gorbachev developed his arguments for an audience of distinguished scientists and academics, invoking the names of Albert Einstein and Niels Bohr in appealing for peaceful uses of science.

By focusing on SDI and the U.S. military, Gorbachev injected a note of warning about the dangers of failure in Geneva, but by his more positive statements, he appeared to make the case that if the summit fails, it should not be seen as the Soviets' fault.

"Soviet people in their letters often ask what the Soviet Union will do if the United States, in spite of everything, embarks" on SDI, he said. "We have already said that the U.S.S.R. will find an effective answer . . . but if this happens, the case in point will be a new round of the arms race."

Gorbachev today gave a graphic description of "Star Wars," painting what he said was a picture of the world in 10 or 20 years. "Waves from all manner of strike weapons will be rushing overhead everywhere, from the edge of the atmosphere at an altitude of 100 kilometers to geostationary orbits, above all people inhabiting our planet," he said.

"I think that tension in relations between our countries will escalate to a point unprecedented even by today's standards and be even more difficult to control," he said.