Polish authorities today accused a U.S. diplomat here of "extradiplomatic" contacts with "antistate opposition" and showed film of him on television in an escalation of a month-long series of public attacks on the Reagan administration.

State-controlled television and the official news agency PAP accused Stephen Donald Mull, a second secretary in the political section of the U.S. Embassy here, of being "a staffer of U.S. intelligence" who had met with various opposition figures as well as with an accused spy named as Bogdan Charyton, a Polish citizen. PAP said that Charyton had been arrested and had acknowledged passing on sensitive information to American intelligence.

The official reports made no mention of any investigation of the U.S. diplomat or action against him. However, in an unusual step, state television showed film of Mull walking out of the embassy and meeting with several opposition figures. A U.S. Embassy spokesman had no immediate comment.

The accusations were the second television attack on the United States in two days and added to what western diplomats here have described as a growing anti-American propaganda offensive since early May. The campaign has unfolded in the weeks before an important communist party congress and has signaled both the poor state of U.S.-Polish relations and an effort by communist leader Wojciech Jaruzelski to appeal to party hard-liners, the diplomats said.

"There are elements here that don't think it's worth trying to improve relations, and this is meant to please them," a senior western diplomat said. He added: "It's been an irritation for Washington."

The new official hostility has reversed a slight warming trend in U.S.-Polish relations marked by the visit here in March of former U.S. ambassador to Poland Walter Stoessel, the highest American official to meet with Jaruzelski since 1981.

Since then, Polish officials have been disappointed by the failure of the Reagan administration to lift continuing economic sanctions against Warsaw, while U.S. officials say they have waited in vain for Jaruzelski to respond to the visit with a gesture of his own.

Recent developments, said the western diplomat, "have tended to block anyone trying to move in a positive sense on Polish relations." He added that "the attitude in Washington is to wait and see what happens in the next month."

The most striking Polish attack on the Reagan administration came earlier this month, when government spokesman Jerzy Urban, in an interview with The Washington Post, volunteered confirmation that a CIA spy on the Polish Army's general staff had escaped with plans for the imposition of martial law and outlawing of the Solidarity trade union in the fall of 1981.

Urban argued that the United States could have prevented martial law by warning the world of the Polish government's plans and said the incident proved the Reagan administration sought upheaval in Poland. The State Department dismissed this charge, but the Polish official media have kept the story alive with a stream of follow-up articles and commentaries.

Urban has also made frequent statements since last month about an officially backed plan to send thousands of sleeping bags and blankets to New York for the city's homeless. The initiative has been accompanied by extensive media reports about the problem of homelessness in the United States and the Reagan administration's alleged lack of concern for the poor.

Other public-relations moves in the last two weeks have included the preparation of a new "white paper" on the government's estimate of damage caused by U.S. economic sanctions against Poland, imposed since 1981, plans by state television to broadcast a film depicting pornography and eccentric behavior by Americans, and a charge by Urban that the U.S. Embassy may have known the whereabouts of Solidarity underground chief Zbigniew Bujak before his arrest May 31.

Some Polish sources have explained the campaign as part of the government's effort to prepare a favorable domestic climate for this month's party congress, the first since Jaruzelski assumed the communist leadership in 1981. Though Jaruzelski's control over the party has not appeared at risk, the general has appeared to cater a number of recent statements and media iniatives to hard-line dogmatists who have opposed his policies.

"It's show-and-tell time for Mikhail Gorbachev," added one western diplomat, referring to unconfirmed reports that the Soviet leader will attend the congress. "Jaruzelski wants to show that he's in control and has blocked the conservative elements in the party."

At the same time, diplomats here noted that the government offensive was at odds with a notable warming trend in Poland's relations with other western countries and its recent admission to the International Monetary Fund after a 35-year absence.

Recent statements by officials have suggested a growing view that Poland should concentrate on improving political and economic relations with Western Europe while writing off the Reagan administration. In this sense, the media attacks may signal a long-term change in political strategy, several observers said.

In a related development, Solidarity founder Lech Walesa was summoned for police questioning next week shortly after he applied for a passport to travel to Italy, United Press International reported. Dissident sources said Walesa was to be questioned on possible links with Bujak.