Polish leader Stanislaw Kania committed the ruling Communist Party to major internal reforms today, despite apparent misgivings in Moscow, and demonstrated convincingly that the balance of power within the Polish party has shifted significantly in favor of the reformers.

In a speech to the party's Central Committee, Kania said it was impossible for Poland to overcome its political and economic crisis without an overhaul of the Communist Party itself. He endorsed proposed changes in the party's statutes including democratic elections by secret ballot, freedom of debate, and limitations on the term of office.

For the first time, he also proposed a specific date for an extraordinary party congress, which is likely to result in sweeping changes in the leadership and the adoption of a new program. Subject to the Central Committee's agreement, the congress will take place from July 14 to 18.

Radio Warsaw reported early Thursday that the Central Committee lifted the Poliburo membership of former premier Josef Pinkowski, who had been a member only since Poland's labor and economic strife began last summer, news services reported.

[Three others also were stripped of their membership in the ruling group, and two workers -- neither of them members of the trade union movement Solidarity -- were elected to the Politburo. Pinkowski was replaced as premier in February.]

Kania sweetened his speech for Moscow by underscoring Poland's obligations to its communist neighbors and thanking the Soviet Union for its "enormous" economic assistance. The official Polish view is that reforms will strengthen rather than weaken the Communist Party's grip on society.

It was not immediately clear whether Kania's speech would satisfy the expectations of rank-and-file party members. At today's meeting, some local communist organizations demanded representation of the new so-called "horizontal" groups formed by party members dissatisfied with the traditional "vertical" structure in which authority flows from the top down.

Radio Warsaw reported today that a group of rank-and-file activists were barred from attending the meeting as observers. But the party leadership came under fire from disgruntled members who charged that the process of change was being delayed and who called for dismissals of a number of Central Committee and Politburo members.

It was a national conference of these "horizontal" groups two weeks ago that had alarmed Moscow more than other aspects of the Polish crisis, which started with a workers' revolt last August and brought about dramatic changes in the Polish society. Apart from bringing down a government, Polish workers succeeded in forcing communist authorities to recognize the existence of independent industrial and farm trade unions.

Soviet concern at recent developments in Poland was illustrated by the brief visit here last week of the Kremlin's chief ideologist, Mikhail Suslov. Shortly after his departure, the Soviet news agency Tass issued an ominous warning against "revisionist" elements in the Polish Communist Party -- one of the most damning epithets in the Marxist-Leninist lexicon.

Throughout the crisis, Kania has projected himself as a politician of the center. Torn between pleasing Moscow and pleasing the Polish nation, he has tried to steer a middle course. His strategy has been one of controlled reform.

Today's Central Committee meeting was a sequel to the stormy session a month ago when the leadership came under sharp attack from workers' representatives. The inconclusive results of that meeting bitterly disappointed rank-and-file Polish Communists who had been hoping for immediate changes at the top.

"We want to emphasize that the key to overcoming the crisis is the party. Renewal in society is impossible without a renewal in the party as well," Kania said. He was careful, however, to specify the limits of reform in two crucial areas. Nominations to all important administrative posts in the country, he said, would remain the prerogative of the party and controls over the mass media would continue.

Yesterday, Polish Politburo member Kazimierz Barcikowski said the national conference of "horizontal" groups had undermined "both the party and the confidence of our friends in our ability to resolve our crisis."

Today, however, Deputy Premier Mieczyslaw Rakowski said the horizontal groups should have been assimilated into the official party structure long ago. He said Poliburo members had not drawn the correct conclusions from recent meetings with ordinary party members and described the Central Committee as currently constituted as "unable to lead the country out of the crisis."

The present Central Committee was elected at the last party congress in February 1980 when delegates were carefully chosen for their loyalty to former party leader Edward Gierek. One of the reasons why reformers in the Polish party have been pressing so hard for the holding of an extraordinary congress is that it will provide an opportunity to elect a new leadership.

Meanwhile, a former strike leader who smuggled himself back into Poland after eight years in exile voluntarily reported to the public prosecutor. Edmund Baluka was charged with cooperation with Radio Free Europe and illegal entry into the country. Under a compromise reached earlier between the independent trade union Solidarity and the authorities, Baluka wil remain free while carrying out his legal defense.