Polish Communist Party leader Stanislaw Kania today described Soviet fears about developments in Poland as "fully justified" and the nation faced one of the most dramatic moments in its 1,000-year history.

In a carefully balanced speech replying to a toughly worded letter demanding a halt to liberalization in Poland, Kania, nevertheless, made it clear that under his leadership the Polish Communist Party would stick to the main elements in its reform program. He also said it was the opinion of the ruling 11-member Politburo that an extraordinary party congress should be held as scheduled in mid-July.

Kania's speech, which was made to a hastily assembled meeting of the Polish Communist Party Central Committee and broadcast on state radio, marked the first official acknowledgement of the existence of the Soviet letter, which was delivered to the Polish leadership last weekend. Political analysts here believe it was provoked by the approach of the congress and the knowledge that the meeting could result in sweeping changes in the Polish leadership and the adoption of major political and economic reforms.

Making clear his personal commitment to solving the crisis by political methods rather than force, Kanis said the Politburo believed there was "no sensible alternative to the policy of "socialist renewal," as the party's reform program is dubbed here.

There were signs, however, that some members of the 140-man Central Committee favored a new, tougher line against the independent trade union Solidarity to avert the possibility of a Soviet invasion.

A speaker from the northern town of Torun, Zygmunt Najdowski, advocated a change in strategy from "resolving the crisis by peaceful means and by our own forces" to "resolving the crisis by our own forces at all costs."

This new formula apparently signified a readiness to consider the use of the Army and police to put down future labor unrest. It is widely assumed here that such a change of political course would inevitably involve a change of leadership, and Najdowski went on to call for changes in the Politburo.

There was, however, no immediate indication of the strength of support for Najdowski's views.

The meeting was adjourned after a debate lasting seven hours and will be continued Wednesday to consider a final resolution and possible personnel changes.

Meanwhile, an unofficial text of the Kremlin letter, which was signed by President Leonid Brezhnev on behalf of the Soviet Central Committee, was seen by Western journalists for the first time. Even tougher than orignally thought, it states that the Polish leadership failed to follow Soviet advice on the necessity for strengthening the Army and security services to counter the activities of "antisocialist forces."

The result, it adds, has been "the disarmament of the socialist state and its abandonment to the mercy of the class enemy."

The letter states that the situation within the Polish Communist Party has become a source of "particular concern." It expressed alarm at the election of what it called "opportunists" as delegates to the congress and added: "One must not exclude the possibility that during the congress itself an attempt could be made to strike a decisive blow against the Marxist-Leninist forces in the party and to liquidate the party itself."

The document does not explicity call for the postponement of the congress. But is does say that the Polish party should find within itself forces strong enough "to reverse the course of events . . . even before the congress." s

The debate at today's Central Committee plenum reflected the huge dilemma facing the Polish party leadership. Internally, the pressure for reform and greater democracy has become virtually unstoppable. Externally, Poland's Soviet-bloc neighbors are becoming more concerned at what they regard as a threat to the entire socialist community.

Ever since he succeeded the now disgraced Edward Gierek as Communist Party leader in September following a crippling wave of strikes, Kania has sought to steer a middle course between the expectation of the Polish nation and the political and ideological constraints imposed by Moscow. But the balancing act is becoming increasingly difficult to maintain.

One small piece of good news for the Polish leader was the fact that a two-hour warning strike scheduled for Thursday in the northern town of Bydgoszcz appears to have been avoided. Solidarity's national leadership and government negotiators have appealed to local union branches to suspend the strike, which was called to protest the failure to name officials respondsible for beatings of union activists in the town in March.

Clearly addressing himself to Moscow, Kania underlined the fact that Poland has enjoyed a period of relative labor peace during the past two months. But he expressed great concern at what he called a rise in anti-Sovietism, the lack of social discipline and actions against the militia.

These were all specific complaints raised by the Soviet party in its letter, copies of which were distributed to all Polish Central Committee members and other leading officials in advance of today's meeting.

Replying to Soviet criticisms of relaxation of party control over the mass media, Kania acknowledged that what he described as "editorial discipline" had been weakened. Privately, editors of Polish newspapers have been told there must be an end to public criticism of the Politburo and the police.

As in Czechoslovakia during the liberalization movement in 1968, Polish newspapers have become much freer and more informative during the last nine months. This, combined with an explosion of uncensored publications issued by the independent trade union federation Solidarity, is clearly one of the Soviet Union's major grievances.

Kania also echoed the Soviet criticism of radicals in Solidarity who, he said, were linked with dissidents from the Committee for Social Self-Defense (KOR). He said attempts were being made to isolate Poland from its allies through the spread of slanderous leaflets and "barbarous acts of vandalism" against Soviet war memorials.

"There can be no toleration of any acts directed against socialism, our alliances, or our friendly relations with the Soviet Union," he warned. "We have to prove this in practice starting from tommorrow."

But other than insisting that Communist Party discipline would be tighened and political activity by Solidarity would be opposed, he gave little indication of any concrete steps that might satisfy the Soviets. The Soviet letter implicitly criticized Kania and the Polish prime minister, Gen. Wojciech Jaruzelski, for failing to make good on repeated promises of firm action against "antisocialist forces" in Poland.

Kania, however, endorsed the Soviet analysis of events in Poland by saying, "The concern anxiety of the fraternal [Communist] parties about development here, fears for the consequences of prolonging the crisis, and worry about the repercussions for the security and future of the socialist commonwealth are fully justified."

Acknowledging the critical nature of the Kremlin letter, he said, "Our friends have full rights to react in this way."