Poland's leaders announced today an emergency meeting of the Communist Party's policy-making Central Committee following persistent reports of a tough letter fromthe Kremlin demanding firm measures to restore the party's authority.

The letter could not be officially confirmed, but usually reliable Polish sources said the communication from the Soviet Communist Party's Central Committee was delivered here Friday. The semiofficial sources, who asked not to be identified, said it verged on an ultimatum and accused the present Polish leadership of going back on promises to reassert party authority over the independent trade union federation Solidarity.

The letter, the sources added, specifically menitioned the Communist Party secretary, Stainslaw Kania, and the premier, Gen. Wojciech Jaruzelski, who have both been identified as the foremost advocates of a concilliatory line toward Solidarity.

The official Polish news agency PAP said only that the Central Committee would meeting Tuesday to discuss the political situation facing the country and preparations for the congress. According to unconfirmed reports circulating here, the decision-making 11-members Politburo held extensive meetings Saturday and today to consider the Kremlin's letter.

The Kremlin is said to have demanded firm measures against what was described as the threat of counterrevolution in Poland and to have criticized a package of reforms that are due to be introduced at a special Communist Party congress next month.

Counterrevolution is a term often used by Communists in connection with events that they believe are threatening a party's control of a country.

Tonight several Western news organizations here received annoymous telex messages alleging that the Polish Politburo had received a Soviet "ultimatum" asking that the armies of neighboring communist countries be ensured the most important lines of communication through Poland. The telex message said the demand was connected with "the loss of confidence in the leadership of the party and government of the Polish People's Republic." t

In all cases, the telex machine went off before the sender could be identified. Polish officials were unable to confirm or deny either report.

The reports, however, fit a pattern of a heightened campaign in the official Soviet Bloc news media against Poland during the last week and the much sharper tone used by Polish leaders in speeches this weekend. In a speech today in the northern town of Bydogoszca, Deputy Premier Meczyslaw Rakowski specifically stated that the patience of Poland's allies was "running out" and that Poland's independence was "in danger."

Such language has been used before at crucial points in Poland's 10-month labor crisis, and there is no evidence of imminent Soviet military moves against Poland. But it is generally accepted here that the political balancing act in Poland is entering a crucial, and perhaps decisive, phase in view of two party congress that is scheduled to open July 14.

There was, however, little visible sign of heightened tension in Warsaw, and Rakowski himself was seen later tonight in one of the capital's plushest nightclubs.

If the congress goes ahead as planned, it could result in sweeping changes in the leadership and the introduction of major political and economic changes. An election campaign is now under way throughout the country for delegates to the congress and a new party program is being prepared.

Contributing to tension here has been the threat of a two-hour warning strike in Bydgoszca next Tuesday over the government's failure to name and punish officials responsible for the beatings of Solidarity activists in the town in March. The strike was called despite the opposition of Roman Catholic bishops who argued that it would violate a 30-day period of mourning for the late primate of Poland, Cardinal Stefan Wyszynski.

In his speech in Bydgoszcz, Rakowski repeated many of the points made in an interview Saturday, including the charge that radicals within Solidarity were trying to provoke a political struggle with the authorities. But he also struck a more ominous note by saying that the "line of agreement" pursued by the Polish leadership since August's strike was in jeopardy.

"The question of power has become an everyday problem in Poland," he said. "This does not mean Jaruzelski's authority, nor Kania's, nor mine, but people's authority. This is in danger. Therefore, Poland's statehood is also in danger. Whoever minimizes this fact in the coming days and weeks will give a poor picture of himself, he will occupy an extremely poor place in history."

Rakowski said that the "limit of possible compromise" open to the authorities had been exploited to the full and that a new campaign had been launched designed to bring the authorities "to their knees."

He added: "This is being done at a time when we are in a terrible plight with no further help from anybody, at a time when the socialist countries are increasingly annoyed and both East and West express exasperation over the fact that they have to feed those who do not want to work."

He said that anti-Soviet attitudes were evident not only in the tearing of red stars off of Soviet military garves, but in articles and cartoons in Solidarity's internal bulletins and various factory papers.

"In the nearest future, Poland can become the hotbed of tension which might reach out beyond its borders," he said. "Does this have to happen? I think not. I still believe there is a chance to fulfill the policy of agreement, though it is dimishing."

Radowski is to hold a meeting with Solidarity leaders in Warsaw on Monday to discuss their demands for the punishment of those responsible for by Bydgoszcz incident.