The reported surrender this week of a principal Polish underground leader has left a trail of riddles, causing many of his colleagues and followers to ask whether Wladyslaw Hardek's decisions to accept the government's amnesty offer and to go on television Tuesday night to denounce opposition activity were made voluntarily.

Why, they ask, would Hardek, who had been a staunch advocate of underground opposition, appear so suddenly to have changed his mind?

He said he made his decision on Monday but then, they ask, why did he join other members of Solidarity's underground national coordinating committee that same day to sign a statement calling on workers to carry out plans for various protest actions this week and next?

Although he said also that his decision to surrender had been "a deeply thought one," why did his colleagues on the underground commission have no apparent inkling of Hardek's second thoughts?

His former above-ground Solidarity associates and his family here have no certain answers to these puzzles, but they express deep doubts about the legitimacy of Hardek's televised message, which shocked supporters of Solidarity.

Whether coincidental or not, there had been increased police activity around Hardek's apartment before his appearance. Security agents stayed overnight in the home two weeks ago, and arrived again Aug. 19 to search the premises. The phone line was cut last weekend, restored Monday, then cut again Wednesday, according to the family.

Observers note that before anticipated major protest actions in October and November of last year, and May of this year, authorities announced the capture of top underground leaders, including Wladyslaw Frasyniuk, Piotr Bednarz and Jozef Pinior, all from the Wroclaw region.

The Hardek episode has provoked disbelief among workers at Nowa Huta's Lenin steelworks, Poland's largest factory, where the 36-year-old Hardek was a foreman in a plate-making shop and had been the first chairman of the factory's Solidarity branch.

"It's a lie," said one young steelworker, echoing statements about the Hardek episode by a dozen other laborers interviewed as they left the millworks Thursday. "He didn't come out of hiding. He was arrested and made to seem to be asking for amnesty."

"No one believes he would have made that statement voluntarily," said a steelworks veteran and Solidarity activist. "Some suspect he might even have been drugged." Another employe said, "The general feeling is, it's a trick."

Several said they doubted that the person on television actually was Hardek--although his family said it was. He did not look the same as he did when he went underground in December 1981 after leading a strike at the steelworks to protest the imposition of martial law. He had grown a beard, donned dark glasses and shed some hair and weight.

Some of the skepticism appears to reflect the government's lack of credibility among many Poles.

Also, another senior Solidarity activist--Krzysztof Wyszkowski of Gdansk, one-time managing editor of Solidarity's weekly paper--reportedly was captured last month while in hiding, then pressured by authorities to ask for amnesty.

Although Wyszkowski said he never agreed to do so, he was reported by the government to have been among the first underground activists to take advantage of the amnesty offer introduced July 22, when martial law formally ended. Wyszkowski accuses the Communist authorities of trying to use him to draw others out of hiding.

Hardek's family confirmed that it was he on television. Hardek's wife, his son, 14, and daughter, 7, saw him briefly in person on Wednesday, when he returned to their apartment in this industrial suburb of Krakow after questioning by police. He stayed only an hour and half, long enough to change clothes and pick up a clean towel and soap, family members said.

He did not speak of what had happened, the family said, and they did not press him--although it was the first time in 20 months that he had come home. He told them there would be time later to talk, then left, saying he would be gone for a few more days to take care of unspecified urgent business.

Several former Solidarity associates of Hardek's at the steelworks described him as a serious person and a committed activist, a political moderate who was a smooth and effective speaker. After setting up the Solidarity branch at the steel plant, Hardek lost the chairmanship of the group in March 1981 elections to a more popular colleague, but he remained active as a member of the regional Solidarity board.

After martial law was imposed, Hardek represented the Krakow area on Solidarity's five- or six-member Provisional Coordinating Committee, the top underground leadership group.

A Nowa Huta steelworks underground bulletin contains a statement by Hardek dated Aug. 9 criticizing the amnesty offer as too restrictive and conditional and calling on Solidarity activists to continue the struggle against a repressive state.

Hardek's signature appeared also on the committee's latest statement, dated Aug. 22--the day Hardek said on television he had decided to surrender--urging workers to join a slowdown this week and reiterating a plan to boycott public transit and official newspapers on Aug. 31, the third anniversary of the signing of the worker-state agreements that gave rise to the now-banned Solidarity.

Zbigniew Bujak, the best known underground leader, issued a statement that reached western correspondents here Wednesday expressing surprise at Hardek's televised surrender.

"Up to the last moment," Bujak said, Hardek "took part in the work of the coordinating committee. At this moment I do not know what circumstances caused him to find himself in the hands of the authorities nor those that led to his appearing before the cameras. Whatever they were, they brought another loss. But as we have done in the past, we will manage to make up for the loss."

Solidarity activists here said the Hardek episode will not affect next week's protest plans, which include wreath-layings in memory of several workers who died during martial-law clashes and a march Wednesday from the steelworks' gates to a Nowa Huta church.

"The lack of one brick in the wall doesn't mean the wall will fall down," said an opposition activist. "Hardek alone wasn't Solidarity."