Poland's Communist Party leader, Stanislaw Kania, today called for a purge of wavering party members in a new attempt to wrest the political initiative from the independent trade union Solidarity.

In perhaps his toughest speech yet to the party's policy-making Central Committee, Kania accused "extremist leaders" in Solidarity of attempting to exploit Poland's economic crisis to seize political power and abolish socialism. He also proposed a temporary ban on strikes and a suspension of the workers' newly won right to "free Saturdays" in key industries during the winter.

The speech, delivered on behalf of the ruling Politburo, reflected a gradual shift of political power in the Polish party toward the hard-line faction as the struggle with Solidarity intensifies. But Kania also reiterated a previous offer to expand the Communist-dominated National Unity Front to include moderate representatives of Solidarity and other trade unions.

The tough rhetoric used by Kania appeared to be at least partly designed to defuse dissatisfaction at the two-day Central Committee plenum with his own conciliatory, but lackluster style of leadership. In what could be a prelude to a stormy debate over personnel matters, a successful attempt was made at the start of the meeting to include on the agenda an assessment of the Politburo's work and the party's internal "organizational" problems.

The Central Committee meeting coincided with an announcement that the regular two-year period of military service was being extended by an additional two months. The official explanation was that the extra number of conscripts would be involved in projects to help tackle the economic crisis such as working in the mines.

While Kania's speech represented an escalation in the rhetorical attacks on Solidarity, it provided few concrete indications of how the party intended to assert itself more forcefully. The plenum was called to formulate a response to Solidarity's national congress, which ended earlier this month with calls for sweeping economic and political changes.

Probably the most significant new point raised by Kania was his call for a purge of the party's own ranks. This appeared to be an effort to force the estimated 1 million Polish Communists who also belong to Solidarity to declare their loyalties once and for all.

Since Solidarity is numerically a much stronger organization than the Polish party, the phenomenon of joint membership has tended to weaken the party more than the union during periods of confrontation. While every third party member and every fifth Central Committee member also belongs to Solidarity, the proportion of union members who belong to the party is only one in 10.

In his speech, Kania said double membership had become a major problem and every local party organization should decide its true identity. It was impossible, he said, "to be at the same time a member of our party and one which is its enemy."

One of the chief Soviet Bloc criticisms of the Polish Communist Party is that it has allowed itself to become a mass organization in which careerists and cynics outnumbered the ideologically committed socialists. This mattered little when all decisions were made by a small clique at the top but, in the Kremlin's view, is dangerous at a time of democratic ferment.

Shortly after Kania's speech, the one Solidarity member on the 15-man Politburo, Zofia Grzyb, announced she was leaving the union because it had transformed itself into "a political opposition." A worker in a shoe factory, Grzyb joined Solidarity soon after its formation in September of last year following massive strikes along the Baltic coast.

Grzyb's resignation from Solidarity followed the expulsion from the party yesterday of Bogdan Lis, the sole card-carrying Communist on the union's national leadership.

Meanwhile talks between Solidarity and government negotiators over price reform continued into their second day. A government spokesman flatly rejected a call by Solidarity for the creation of a socio-economic council with power to initiate new legislation in Parliament and draft measures to counter the crisis.

The spokesman, Jerzy Urban, said on television that the government would never agree to such a body since it would lead to "total dictatorship" by Solidarity and the paralysis of the state apparatus.