In a harsh attack, the Soviet Union today for the first time openly accused the independent Polish trade union federation Solidarity of using "blackmail, threats, provocations and physical violence" to undermine the authority of the Polish Communist Party.

The Tass news agency dispatch from Warsaw is the Kremlin's most direct denunciation of Solidarity since the labor union movement gained official status in Poland last year. It is seen by observers as being aimed at stiffening the Warsaw leadership of Stanislaw Kania as it heads into a new round of negotiations with Solidarity.

"Making use of blackmail, threats, provocations and now, quite often, physical violence, intensifying anarchy in the country, the leaders and extremists in Solidarity are advancing ever more arrogant political demands, placing Solidarity therefore in a position of political opposition to the Polish United Workers Party and state power," the Tass dispatch said. The Polish United Workers Party is the formal name of Poland's Communist Party.

Tass claimed that "oppositionist antisocialist forces have become more active in Poland" since last Saturday's strike, in which Solidarity called approximately 5 million workers off the job in a move to press for a five-day work week and other labor demands.

Tass asserted that Solidarity is using the work-free Saturday issue as a pretext to force the government to release right-wing political prisoners from jail and strengthen antisocialist groupings.

Soviet military units along the Polish frontier have been at a state of high readiness since autumn. As the crisis has unfolded, Moscow has repeatedly made threatening gestures on the eve of important showdowns or negotiations between the party and the workers that seem designed to encourage the party to be tough and to intimidate the workers.

It is throught here that the Soviets will intervene with military power only if the party loses control and if Soviet lines of communication to their forces in East Germany are endangered.

Solidarity "is making a shift to the right, resisting efforts of the Central Committee and the government to normalize the situation and ensure the people's vital interests," Tass said. "Everything shows that the speculation on the question of work-free Saturdays is a tactical method used by Solidarity leaders and the antisocialist forces behind them. They put the question as follows:

"'For the sake of calm, accept our political conditions, in particular release political prisoners, first of all members of the Confederation of Independent Poland', who, as many know, strive for the overthrow of the socialist system in Poland and for the country's withdrawal from the socialist community and the Warsaw pact."

Until now, the Soviet news media have seldom spoken directly on the Polish crisis. Instead, they usually quote papers in Poland or elsewhere in the Eastern Bloc, especially hardline publications in East Germany and Czechoslovakia.

The Tass dispatch quoted an article in Zolnierz Wolnosci, the Polish armed forces newspaper, which recently warned that the strikes aimed to undermine Polish socialism "and bring to power the opposition."

"Using the slogan of socialist renovation as a cover, the counterrevolutionary forces are aiming at the dismantling of socialism in Poland," Tass quoted the paper as saying.