The Polish government accused two U.S. diplomats today of taking part in an antigovernment demonstration yesterday near Krakow, while a municipal court in Warsaw sentenced two prominent dissidents -- Jacek Kuron and Seweryn Jaworski -- to three-month jail terms for their involvement in a protest march here.

The moves reflected tough posturing by Poland's communist government after its official May Day celebrations were upstaged by an opposition rally in the capital, violent clashes between police and demonstrators in Gdansk and detentions in other cities where potential protest actions were thwarted.

A government statement charged two U.S. diplomats stationed in Krakow with "leading" a brief illegal parade in the industrial district of Nowa Huta. The two men -- identified as William Harwood, a first secretary, and David Hopper, a consul -- were detained with a number of Poles, all said to have been "behaving aggressively." The brief demonstration, which took place following a mass at the Church of Our Lady of Czestochowa in Nowa Huta, was reported to have included "antistate calls, banners with hostile inscriptions and leaflets."

The U.S. charge d'affaires in Warsaw was summoned to the Foreign Ministry this afternoon and handed a formal note protesting what Polish authorities termed a "flagrant violation" of diplomatic status and international norms as well as "an unacceptable interference in Poland's internal affairs."

A spokesman for the U.S. Embassy countered that the Polish government was "seriously misinformed" about the actions of the diplomats. In a statement, the spokesman said the diplomats were "not in any way participating" in the demonstration and had been present merely as "observers."

When asked by police to identify themselves, the U.S. diplomats had produced diplomatic identification cards, the spokesman said, but were nonetheless ordered into an unmarked vehicle. One of the men objected, and he reportedly was pushed, hit, kicked and forced into the vehicle.

Upon arrival at a militia station, the diplomats were held for 20 minutes, then released with apologies. A counterprotest against the detention of the men and their alleged mistreatment has been lodged by U.S. authorities.

The incident was the latest in a string of diplomatic clashes between the United States and Poland indicating the poor state of current relations. A commentary carried by the state-run Polish press agency, PAP, this evening recalled the expulsion in February of a U.S. military attache on allegations of photographing an off-limits military installation and said it was "obvious" that the aim of the U.S. Embassy here is "by no means oriented at developing friendly" relations. Rather, PAP said, the embassy's goal is to destabilize Poland, and the Nowa Huta incident was a "political provocation."

In another development, Belgian Foreign Minister Leo Tindemans postponed an official visit to Poland next week because he will not be allowed to meet with Solidarity members, a Foreign Ministry spokesman in Brussels said.

Spokesmen for the government declined today to give figures on the number of people detained in connection with yesterday's protests. In Warsaw, observers at the misdemeanor court counted more than 30 persons being punished on charges of disturbing the public order. Most of them received fines well above an average worker's monthly salary.

The heaviest sentences went to Kuron, a former communist party member who founded the Committee for Social Self-Defense, known as KOR, in 1976 and advised senior leaders of the now-outlawed Solidarity union, and to Jaworski, a one-time steelworker who was deputy chief of Solidarity's Warsaw branch.

It was the negotiating efforts of Kuron and Jaworski, who met with police when yesterday's march was trapped by militia lines, that achieved the peaceful dissolution of the rally. Jaworski had been at the front of the procession, which numbered at least 15,000 chanting and banner-carrying Solidarity supporters, while Kuron appeared only at the end to bargain with the police.