Poland's Communist Party leader, Stanislaw Kania, warned today of an unprecedented "national tragedy" unless the country finds a way to halt a wave of strikes and public marches over food shortages.

Addressing the policy-making Central Committee, the party leader said a new and dangerous element has entered the Polish crisis as a result of street demonstrations organized by Solidarity, the independent trade union federation. The government, he said, has to mobilize its forces to contain the crisis and protect the living standards of ordinary Poles.

The Central Committee plenary session was the first since last month's extraordinary Communist Party congress -- and therefore an important indicator of the outlook of the newly elected leadership. The speeches suggested a hardening resolve toward Solidarity and a determination to assert the party's authority.

The tone was struck by Kazimierz Barcikowski, speaking on behalf of the Politburo, who accused Solidarity of conducting a slanderous propaganda campaign against the government, of paralyzing the economy and undermining Poland's position abroad.

"Recent events prove that Solidarity is proceeding along an adventuristic road and we must be prepared for further actions aimed at weakening the authorities and gradually taking them over," he said.

Since the congress, the Polish party has come under increasing pressure from its Soviet Bloc allies for a tougher line. According to an unconfirmed report, the leaders of Poland's three communist neighbors -- the Soviet Union, Czechoslovakia and East Germany -- have sent new letters to the Polish party calling on it to reassert its authority. Couched in similar language, the letters are said to have expressed readiness to provide technical or material assistance in this operation.

The information came from a usually reliable Polish party source, who said the letters arrived last Friday. The report could not, however, be confirmed officially.

A strongly worded letter from the Soviet party criticizing Polish leaders for weakness in the face of alleged "counterrevolutionary" threats was delivered here in early June and later published in the official press. It was followed by similar messages from other Communist parties, the contents of which were not disclosed.

The Soviet commander of the Warsaw Pact defense alliance, Marshal Viktor Kulikov, arrived here last weekend after earlier visits to Czechoslovakia and East Germany. A frequent visitor to Poland in the last few months, Kulikov held talks with Premier Wojciech Jaruzelski and watched military maneuvers in the southern Silesian region.

The exercises in Silesia, which are being attended by Czechoslovak and Soviet units in addition to Polish forces, coincided with a big Warsaw Pact naval exercise in the Baltic. Western diplomats here said the land exercises appeared to be on a small scale and the overall level of military activity seemed considerably lower than in March and April, when the possibility of a Warsaw Pact military intervention was widely discussed in the West.

Kania's address to the Central Committee today marked his first appearance in public since a brief illness forced him to remain at home and cancel official engagements. Stressing that the situation is difficult and tense, he said: "We must find a way of ensuring that Polish streets are peaceful again or the logic of developments could lead to the greatest-ever national tragedy."

As the 200 members of the newly elected Central Committee met in Warsaw, Solidarity's national coordinating commission held a second day of talks in the Baltic port of Gdansk. The discussion, which was held behind closed doors, centered on whether to agree to a draft joint communique with the government covering a range of issues from improved meat rations to union access to the mass media.

The problem facing Solidarity leaders is to persuade their 10 million members to respect any agreement reached with the authorities. Most initiatives for the demonstrations and strikes of the last week have come from ordinary workers dissatisfied with deteriorating living conditions, cuts in food rations and ever-increasing shortages of basic consumer items from meat to detergents to coffee.

If Solidarity and the government fail to reach a compromise, it is possible that they will ask the Roman Catholic Church to mediate. Poland's new primate, Archbishop Josef Glemp, has already made clear he is prepared to act as an intermediary should both sides ask him.

The Solidarity leaders also discussed whether to go ahead with a planned march converging on Warsaw next week from several provincial towns -- an attempt to pressure the government into releasing political prisoners. The government has said it will do everything in its power to prevent the protesters from reaching the capital, including blocking off of roads leading to Warsaw.

Yesterday Jaruzelski described the march as "one more effort to incite tension and unrest" and "a threat to peace and public security."

Although the present crisis was sparked by food shortages, it has taken on the character of yet another test of wills between Solidarity and the communist authorities. Following its congress, the party feels stronger and more legitimate -- and its leaders are eager to prove their ability to control events.