The Soviet news agency Tass sharply criticized President Reagan today for his pledge to proceed with his Strategic Defense Initiative missile defense system and charged that Reagan had made "slanderous attacks on Soviet policy" in his news conference yesterday.

Tass called Reagan's position "unconstructive" and said he "reiterated inventions about a U.S. lag behind the U.S.S.R. in the military field, and . . . accused the Soviet Union of being reluctant to negotiate."

U.S. and Soviet negotiators arrived in Geneva earlier this week for a third round of negotiations in the arms control talks that is scheduled to start Thursday.

Tass' critical reaction to Reagan's firm stance on SDI, a space-based antimissile system that is also known as Star Wars, follows a campaign in the Soviet press charging the United States with being responsible for "deteriorating" relations between the two superpowers in the run-up to the summit meeting between Reagan and Soviet leader Mikhail Gorbachev, scheduled for Nov. 19-20 in Geneva.

The Soviets have consistently portrayed an "inflexible" stand on SDI and Washington's recent test of an antisatellite weapon as actions by the United States that negatively influence the presummit atmosphere.

Western diplomats in Moscow regard the increase of Soviet criticism against the United States and its concentration on the SDI program as part of a Soviet strategy to pressure Washington into changing its unyielding plans to pursue SDI research.

"It is a strategy" one western diplomat said, "to give the Soviet Union an image of flexibility and the U.S. an image of inflexibility going into the summit."

The Communist Party newspaper Pravda, in an editorial yesterday, criticized the successful U.S. test of an antisatellite weapon last week.

"The Soviet Union has repeatedly urged the American administration to weigh the negative implications of the testing of antisatellite weapons for the prospects for the Geneva negotiations," Tass said.