Members of the Communist Party Central Committee today received copies of a letter from the Kremlin that, according to informed sources, sharply criticizes the Polish leadership for losing control over the media, failing to prevent anti-Soviet actions and allowing the socialist system to be endangered here.

Semiofficial sources said the letter, delivered from Moscow Friday, demands a halt to internal reforms undertaken here since last summer's labor unrest that led to the creation of a large and powerful independent trade union movement.

An emergency meeting of the party's 140-member Central Committee has been called for Tuesday to discuss the letter. While news of the letter began to filter out over the weekend, when it was discussed by the 11-member Politburo, details of its content -- and the fact that it was signed by Soviet President Leonid Brezhnev -- did not emerge until today.

Meanwhile, the government opened negotiations with the independent trade union federation Solidarity on a threatened two-hour "warning strike" on Thursday in the northern town of Bydgoszcz. The strike was called to protest the govenment's failure to name officials responsible for the beatings of union activits in March, but Solidarity officials said there was a good chance that it will be called off in view of the latest crisis.

It is assumed here that the Kremlin must have calculated that the best chance of influencing developments in Poland lay in addressing the Central Committee directly before a special congress of the Communist Party is held next month. The present Central Committee is considered much more conservative than the party as a whole, but as many as two-thirds of its members are likely to be voted out of office in the national election for which campaigning has already begun.

Some observers regard the Kremlin letter as another, albeit threatening, move in the long war of nerves against the Polish reformers. Others believe that it amounts to a virtual ultimatum to the Warsaw leadership to restore its authority.

Whatever Moscow's intentions, the Central Committee is under enormous domestic pressure not to reverse the present reforms. Any major change of course would undoubtedly trigger a revolt from the vast majority of the 2 million rank-and-file Polish Communists.

The consensus is that the Soviets will wait to see the effects of the letter before coming to any decision on whether to attempt to resolve the crisis by military force. They are believed to be most reluctant to take such a decision, and there is no evidence that an invasion is in any way imminent.

A revealing glimpse of the national mood was provided by an open letter released today by 22 leading Polish intellectuals including the president of the journalists' union, Stefan Bratkowski; the film director, Andrzej Wajda, and leading advisers to Solidarity.

The intellectuals warned that any attempt to halt "the renewal," as the move toward greater democracy is termed here, would provoke massive protests and have "incalculable consequences" for Poland, the Soviet Bloc, and Europe.

Polish analysts believe the main aim of the Kremlin letter is to influence Communist Party policy rather than to secure immediate changes in the leadership. The semiofficial sources described the letter as specifically criticizing both the Communist Party leader, Stanislaw Kania, and the premier, Gen. Wojciech Jaruzelski, for failing to take firm measures to control events despite repeated promises.

Writing on behalf of the Soviet Central Commitee, Brezhnev reportedly listed a series of Bilateral meetings and Soviet Bloc summits dating back to shortly after the wave of labor unrest last year that resulted in the legalization of independent trade unions in Poland. The Soviet party, he is said to have added, had been "alarmed" at developments in Poland for many months but had addressed its expressions of concern to the Polish leadershp rather than the whole Communist Party out of respect for Poland's sovereignty.

The letter was described as saying the Polish reform movement was no longer concerned with the correction of "past errors," but with "continuing and growing concessions" to the enemies of socialism in Poland. It reportedly condemned what it described as Solidarity's extremist wing and imperialist interference in Poland's internal affairs.

Among the specific accusations reportedly raised by Moscow was that the Polish Communist Party was allowing "antisocialist forces" to gain ever-greater influence over the mass media. The fight for socialism could not be won, Brezhnev said, without control over the press.

Turning to the present election campaign in the party, according to sources, the letter said that a growing number of delegates now being chosen for the congress were "revisionists and opportunists."

"Good comrades are being swept aside," it is quoted as saying.

Brezhnev also reportedly complained of anit-Soviet campaigns and defamatory publications in which the Soviet Union was accused of exploiting Poland.Nationalist tendencies were increasing and reactionary forces outside the country were exploiting the crisis in an attempt to change the postwar status quo in Europe, the letter reportedly said.

"The Polish party has responsibilities not only to Poland, but to the entire socialist community," the letter is quoted as saying.