A Polish Communist Party leader today warned that the Soviet Union might cut off vital economic assistance to Poland unless there was an end to alleged anti-Soviet agitation in the country.

The warning, delivered by Politburo member Stefan Olszowski on nationwide television, coincided with the arrival in Warsaw of a high-powered Soviet delegation to discuss economic ties. It was the most explicit official statement yet of the Soviet Union's ability to use an economic weapon against the independent trade union Solidarity.

Olszowski, who is widely regarded as the mastermind behind the Polish government's current propaganda offensive against Solidarity, combined his threat with a conciliatory gesture. He called for the creation of a broad political coalition bringing together representatives of the Communist Party, the Roman Catholic Church and all trade unions including Solidarity.

Olszowski did not specify what political powers the new Front for National Unity would be given, nor whether the union would be allowed a place in government. Last week a senior Solidarity adviser called for a coalition government in which power would be shared more or less equally by the party, the church and Solidarity until free elections could be held.

The variant suggested by Olszowski is unlikely to be acceptable to Solidarity since it would find itself relegated to a minor role without real power. The fact that he publicly aired the idea, however, can be seen as an attempt to reduce the psychological tension in Poland a notch after a week of mounting pressure.

The thrust of Olszowski's speech was that the Polish people were faced with a choice. To allow Solidarity "extremists" to have their way would lead to political disruption, economic chaos and even civil war. But there was also a good possibility that this could be avoided if society rallied around the party's own program of reforms.

Olszowski drew a distinction between the bulk of Solidarity's 9.5 million membership and "a part of the union's leadership" whose aims he said went beyond those of normal political opposition.

"Their goal is a complete change of the social system in Poland and the tying of our country to the West. It is a counterrevolutionary line....Let nobody be surprised at the government's determination to use all possible means to defend socialism."

Turning to economic problems, he said Poland's existence depends on a lasting alliance with the Soviet Union. The Polish economy could not function without subsidized Soviet deliveries of oil, iron ore, cotton and other raw materials.

"We must understand that all anti-Soviet agitation could lead to a situation where our closest ally, a great socialist country, may start wondering whether it should help us any further or merely maintain balanced economic relations with us," he said.

According to semi-official Polish sources, the Kremlin has threatened several times to reduce economic assistance to Poland unless firm action is taken to curb Solidarity. There are reports that the Soviet ambassador, Boris Aristov, delivered such a warning to the Polish leadership at the same time as a protest note condemning the rise of "anti-Sovietism" in Poland.

The published version of the note hinted at Moscow's readiness to use the economic weapon by describing the moral right of Soviet workers to express indignation at events in Poland in view of Soviet subsidies to the Polish economy.

Western economists are divided over the likely effectiveness of Soviet economic measures against Poland. Some argue that, in view of the close relationship between the economies of the Soviet Bloc, it would not be in Soviet interests to add to economic chaos in Poland.

It is clear, however, that the economic weapon is now under serious consideration in Moscow. This added to the significance of the arrival here today of a Soviet economic delegation led by the planning chief, Nikolaj Baibakov who started talks with the Polish prime minister, Gen. Wojciech Jaruzelski.

In his television address, Olszowski said that if the Kremlin were to insist on balanced trade Poland's projected imports from the Soviet Union next year would have to be cut from 4.4 billion rubles to 2.7 billion. The ruble is worth about $1.40 at the official rate. This would mean that imports of raw materials and oil from the Soviet Union would have to be reduced by at least 50 percent.

Olszowski added: "The Soviet Union can do without Polish supplies, but Poland cannot do with Russian supplies." He compared Poland to "a little boy sitting on a branch of a tree, holding on to it with one hand and cutting it off with the other."

The real state of Polish-Soviet economic relations is virtually impossible to unravel given the complex web of barter deals, subsidized prices and losses incurred through the use of energy-inefficient Soviet technology. But, according to official figures, Moscow has provided Poland over the past year with 1 billion rubles and $1 billion in credits. In addition, a moratorium until 1985 has been declared on Poland's debts to the Soviet Union incurred during the last five-year plan period.

Meanwhile Solidarity leaders met in Gdansk to consider their response to the latest accusations from the party.