Polish authorities today announced the capture of one of the country's most-wanted underground leaders as the government stepped up a smear campaign against opposition activists to dampen public support for a planned May Day protest.

Police in Wroclaw in southwestern Poland were officially reported to have detained Jozef Pinior, a major underground figure in Lower Silesia and a member of the five-man provisional coordinating committee of the outlawed Solidarity trade union. Earlier this month, the committee held a clandestine meeting with former Solidarity chairman Lech Walesa and issued an appeal to boycott official May Day ceremonies and stage independent protests instead.

No details were released on when, where or how Pinior was caught after eluding security forces for 16 months since the start of martial law. But the timing of the official disclosure by the Polish press agency PAP fit the government's practice of announcing major arrests just before feared protest demonstrations.

The captures last autumn of the two Solidarity activists who held the Wroclaw seat before Pinior on the underground leadership committee also preceded tense national moments. Piotr Bednarz was taken into custody shortly before a planned nationwide strike last November that ultimately fizzled. The arrest of Wladyslaw Frasyniuk was disclosed in early October, a few days before the Polish parliament voted to abolish Solidarity.

Such conveniently timed arrests suggest that, in Wroclaw at least, police have successfully penetrated the underground. It also is suspected that Polish authorities may know where other underground leaders are hiding but have decided for now to let them operate.

One possible explanation for this is that officials may prefer to have the underground led by known figures whose contacts they can track rather than by lower-ranking and possibly more radical activists.

On the other hand, the continued existence of an active and recognized underground leadership fosters a nagging sense of instability and unrest in Poland that the authorities presumably want to erase at the earliest opportunity.

Whatever the truth, Pinior's capture is bound to have a demoralizing impact on his underground colleagues and provide another example for an already intimidated Polish public of the difficult odds facing former Solidarity leaders still in hiding.

Asked for comment on Pinior's detention, Walesa called it "unpleasant news." The former Solidarity chief had been summoned Tuesday to police headquarters in Gdansk for questioning about Pinior's activities but said afterward that he had refused to answer.

Pinior, who served as treasurer of the Wroclaw Solidarity branch, is celebrated for having withdrawn 80 million zlotys (nearly $1 million) from union accounts 10 days before martial law was declared in December 1981. Solidarity supporters say this was done to avoid confiscation of the funds by the authorities.

Reporting that large sums of money were found in Pinior's apartment when he was captured, Polish television tonight aired a program in connection with the arrest entitled "The Last Payoff" that included photos of stacks of Polish zlotys and American dollars said to have been in Pinior's possession. Among other charges, Pinior is under investigation for embezzlement.

Coinciding with word of Pinior's capture, Polish radio broadcast a half-hour program today based on what sounded like a secretly taped conversation between two women who were identified as having close ties to the Wroclaw underground. Speaking in often vulgar terms, the women discussed Pinior's relationship with his girlfriend and aspects of underground life.

Between fragments of the taped conversation were commentaries by radio announcers casting aspersions on the moral character of the Solidarity activists and their associates.

"It's incredible," said one of the official broadcasters, "that a structure based on such people could arouse such interest among western politicians."