Poland's Communist Party leadership, warning of a national tragedy and possible bloodshed, today accused the independent Solidarity trade union of transforming itself into a political opposition and unilaterally breaking the agreements under which it was formed.

In its most bitter attack yet on the 9.5-million strong union, the party Politburo said Solidarity had been taken over by "counter-revolutionary groups" and was receiving technical and material assistance from "the centers of Western diversion." The statement followed a meeting of the 15-man Politburo called to analyze the results of last week's Solidarity congress.

The Politburo statement did not threaten any specific action against Solidarity. Instead, it sought to put the blame for Poland's deepening political and economic crisis on the union in an apparent attempt to win the battle for public opinion.

The attack was seen here as forming part of a long-awaited counter-offensive by the Communist authorities following Solidarity's call for democratic elections and message of support to free trade unionists in Eastern Europe. Solidarity is due to convene the second stage of its national congress Sept. 26 with the drafting of a detailed program and the election of a new leadership.

Over the last few weeks, the apparent polarization of political opinion in Poland has led to a growing perception here that events are moving toward a new trial of strength between Solidarity and the government over the most sensitive issue of all: raw political power. The Politburo statement coincided with the publication of an article by one of Solidarity's leading advisers, Jacek Kuron, calling for the creation of a "committee for national salvation" made up of representatives of the party, Solidarity and Roman Catholic Church to take over the running of the country.

The Politburo statement said that the hopes attached to Solidarity's birth a year ago following massive workers' strikes throughout Poland had been shattered. It accused the organization of "psychological terror" and said it had been taken over by "adventurist tendencies."

It added: "The agreements of Gdansk and Jastrzembie signed in August and September last year have been unilaterally broken. They have been substituted with a program of political opposition which aims at the state's vital interests and means a confrontation threatened with bloodshed."

Solidarity, the statement said, has adopted "a course toward a new national tragedy."

The Politburo attacked in particular a Solidarity message to Soviet Bloc workers, which it described as "a provocation against the allies of socialist Poland, whose cooperation is a guarantee of the country's economic development, integrity, security, and peace." It said the state would use every means to defend socialism in Poland.

The Polish authorities are known to have come under increasing pressure from the Kremlin to take a tough line with Solidarity, and last week the party leader, Stanislaw Kania, had a meeting with the Soviet ambassador, Boris Aristov. It was assumed that Aristov expressed Soviet concern at resolutions passed at the Solidarity congress and urged strong action by the Polish party.

The Politburo has issued other toughly worded statements in the past, and it remains to be seen exactly how it intends to restrict Solidarity's activities in practice. A confrontation could come over a number of issues, ranging from the union's demands for greater access to the mass media to a proposed new law on trade unions to local street disturbances.

Since Solidarity's congress, a wave of protest strikes has subsided but other potentially explosive disputes have cropped up.

One concerns the draft of a new trade union law, now under discussion by the legislature, which has been amended by the government side to Solidarity's disadvantage. One particularly controversial amendment foresees the possibility of outlawing a trade union if it violates its statutes or Polish law.

Solidarity was legalized last October following a long struggle. During talks with parliamentary deputies yesterday, union leader Lech Walesa described the draft agreement as completely unacceptable.

Another amendment would allow the legislature to declare an unlimited ban on strikes in the national interest, in place of the annual two-month ban that was originally envisaged.

Meanwhile, the two sides appear to be even farther apart on the sensitive mass-media issue. Today Solidarity's weekly newspaper threatened not to print this week's issue in protest against the censor's refusal to allow the publication of two major articles.

The articles -- including one by the newspaper's editor in chief, Tadeusz Mazowiecki -- commented on Solidarity's congress and message to Soviet Bloc workers. Also censored was an invitation for a delegation of Soviet workers to visit Poland to see what was happening for themselves.

Kuron's call for a committee of national salvation came in an article for the union's Warsaw news bulletin. Kuron said that the Communist Party and government were paralyzed and were no longer able to run the country. He said that, because of the threat of Soviet intervention, Solidarity should not attempt to take over political power, but instead should urge the creation of a committee for national salvation made up of representatives of the party, Solidarity and the Roman Catholic Church.

Analyzing the political situation, Kuron said there was a growing feeling of helplessness, since the old system of government had collapsed and none had been put in its place. This led to popular frustration and a growing polarization of society, with open calls for the overthrow of the Communist regime.

Kuron said Solidarity should concentrate on securing a thorough reform of the state and economy through the implementation of self-management in industry and the holding of elections to local government bodies.

The church, meanwhile, issued an appeal to both sides for moderation. A message from the Polish Episcopate said that petty confrontation should be avoided and all Poles should remember that their first duty was to their motherland.

The statement cited the latest encyclical of Polish-born Pope John Paul II, which, it said, had lessons for both sides.