In an open letter to Polish authorities, workers at 13 enterprises in the northern port region around Gdansk have protested a "growing campaign of slanders and accusations" in the official press against former Solidarity union chairman Lech Walesa and the area's activist priest, Henryk Jankowski.

"We state solemnly," the letter reaching western reporters today said, "that we will oppose with all means available to us the tricky and unethical attempts to deprive Lech Walesa and Father Henryk Jankowski of their good name and our trust.

Addressed to the Polish parliament and the Council of State, the statement followed a series of searing attacks on the former union leader and his associates by the official press.

Government officials are deeply annoyed with Walesa for continuing to campaign against Poland's new trade union structure. His repeated pleas for negotiations with the authorities have been rebuffed by a Warsaw leadership that no longer recognizes Solidarity and maintains that Walesa mismanaged the now-outlawed union's opportunity for a national accord in 1981.

In other developments, a Warsaw police spokesman announced that the prosecutor's office had begun investigating the death of a 19-year-old son of a Solidarity sympathizer. Members of the youth's family blame his death on abdominal injuries they suspect he suffered during a police beating.

A police communique acknowledged that the boy, Grzegorz Przemyk, was detained Thursday. The statement indicated that he and friends were picked up for drunken brawling in Warsaw. Family sources said Przemyk and three friends had been stopped by police as they left a wine bar where they were celebrating their high school graduation.

The communique said the police were prompted to call an ambulance to rush the boy from the station to a clinic as a result of "his aggressive behavior and his bodily injuries." What injuries or how he received them were not specified. In the ambulance, Przemyk was aggressive and force had to be used, the statement said.

He was later released at the insistence of his mother but was admitted to another hospital on Friday where he underwent surgery for internal injuries. He died Saturday.

The letter objecting to the slander campaign against Walesa and Jankowski was signed by laborers at the Lenin Shipyard in Gdansk, cradle of Solidarity, as well as workers from a dozen other building and repair yards in Gdansk and neighboring Gdynia. It was dated May 13.

The Polish press has gone on the offensive against Walesa with articles suggesting he is an opportunist and egotist and a puppet of western propaganda.

In recent days, stories about Walesa have included:

* Excerpts from an alleged taped conversation between Walesa and his brother while the former union chief was interned last year. Walesa is reported to have criticized the Roman Catholic Church for not backing his nomination for the Nobel Peace Prize and for not doing enough to save Solidarity after having made a number of gains during the Solidarity period. He also is quoted as referring to himself as a descendant of Caesar. At another point he is said to have discussed making a deposit of $1 million in the Vatican bank.

* A commentary in the Polish Army daily on the alleged misuse of Solidarity funds reporting that Walesa commissioned the union's press office to write a book about him, authorizing a contract with the author for a fee of $118 per printed page and $470 for collecting the necessary materials.

Last week Walesa's friend, Jankowski, came under blistering fire from the local paper Glos Wybrzeza. It accused him of presiding over a "pushy money-grubbing, politically motivated Solidarity altar" and made insinuations that Jankowski had become a priest to atone for the sins of his parents, who professed to be of German origin during World War II.

"The point is that someone with such a blot on his biography," the paper said, "should not engage in political activities.