The Soviet Union today seized on President Reagan's statements Tuesday that a limited nuclear exchange was possible, reaffirming its own position that any such exchange "would inevitably assume worldwide character."

The Soviets advanced new arguments in their propaganda drive to stiffen West European opposition to deploying new American medium-range nuclear missiles as planned.

The official press agency Tass suggested that Reagan believed "a kind of Rubicon on the way to nuclear catastrophe" existed in which only strategic missiles would lead to a major confrontation.

"Those who possibly hope to set ablaze the nuclear powder keg while themselves sitting snugly away from it should not entertain illusions," it said, scoring "the absurd assumption that . . . the Soviet Union will follow nuclear war scenarios worked out in Washington."

It quoted Reagan as saying Tuesday, "I could see where both sides could still be deterred from going into the exchange of strategic weapons if there had been battlefield weapons, troop-to-troop exchanges there." It also quoted him as saying that an "exchange of nuclear blows" in Europe need not endanger the continental United States.

President Leonid Brezhnev said last week that a limited nuclear war would not be possible.

A flurry of commentaries also sharply attacked U.S. claims that the Soviet Union had used chemical weapons in Southeast Asia and Afghanistan.

Referring to Senate hearings on the subject, Tass said that the administration had not produced either direct or indirect proof for its claims.

At the same time, it continued, the United States is sabotaging talks in Geneva on a treaty to prohibit production and stockpiling of chemical weapons while building up its own chemical weapons in Europe, Japan and the Pacific Ocean. Moreover, the administration has earmarked $2.5 billion to develop new chemical weapons, it said.

Another Tass dispatch said without elaboration that "grenades with toxic agents made at factories in Pennsylvania" had been seized "from bandits infiltrated into Afghanistan."

Also today, the Soviets denied that their submarine that ran aground on Sweden's south coast two weeks ago was carrying nuclear weapons.

The statement by Tass coincided with a formal Soviet rejection of Sweden's protest as being "devoid of any legal and factual grounds."

In a note handed to Swedish Ambassador Carl de Geer, the Soviet government accused Sweden of "distorting facts" and reaffirmed its contention that the Whiskey-class vessel had gotten lost. The note, distributed by Tass, did not mention the nuclear weapons issue.

These were only the second public references by the Soviets since their submarine ran aground near Sweden's Karlskrona naval base Oct. 27.

"Hostile reaction in Western circles was so disproportionate to the incident that it can only be qualified as politically motivated invention," the commentary said.