The Soviet Union is portraying the explosion of the space shuttle Challenger as an example of what it sees as the risks involved in space defense research and to reiterate its position that space should be used solely for peaceful purposes.

The Communist Party newspaper Pravda and other official publications have noted, with a tone of dismay, that the incident has not deterred backers of the Reagan administration's proposed space-based antimissile system popularly known as "Star Wars."

"Although it seemed that the tragic spectacle of the public death of people should underline the extreme need to unite man's creative efforts in the difficult cause of the peaceful use of space," Pravda said today, "in Washington there are officials who find it possible to call for the speedy militarization of outer space."

"In the first hours after Challenger became a fireball before millions of TV viewers," the newspaper said, "some legislators were hasty to say that the catastrophe would in no way affect the most dangerous program of Star Wars," officially called the Strategic Defense Initiative. The Soviets have criticized the space shuttle in the past as a testing lab for SDI.

While chiding the United States about the dangers of space weapons, Soviets also reacted to the disaster by expressing sympathy and regret, both publicly and privately.

Pravda published the text of Soviet leader Mikhail Gorbachev's telegram of condolence to President Reagan on its front page.

Soviets approached some Americans in Moscow to express their regret, and particularly for the loss of the crew.

But following the accident, Soviet criticism of space-research dangers has been pointed, and the Challenger has emerged as a case in point.

"The Challenger tragedy," said the newspaper Sovietskaya Rossiya, "clearly showed that space must be an arena of peaceful cooperation between people and that alone."

Several western diplomats in Moscow noted, however, that official criticism of the Challenger and its connections to space research were muted.

The reason, they said, was that Moscow seemed anxious that the accident not create a rift at a time that there appears to be the possibility of improving relations.