A newspaper today announced the first casualty from May Day clashes between Solidarity union supporters and police by reporting the death of a 29-year-old man in the southern steelmaking town of Nowa Huta.

Lech Walesa, the leader of the banned union, from his home in Gdansk hailed the pro-Solidarity rallies as a success but said street battles would not solve Poland's problems. He appealed anew to Communist authorities for talks.

But the officially controlled press indicated no change in the Polish leadership's line, dismissing the May Day counterdemonstrations as a relatively insignificant showing.

In a separate development, armed police this afternoon sealed off the American Embassy in Warsaw to Poles wanting to enter the building. The action followed a formal government protest last week complaining of films and television programs shown by the embassy library that "slander the Polish People's Republic" and demanding an end to the library's "external services."

Teams of police appeared at the embassy's two entrances shortly before 4 p.m., near the time the library was due to show "Period of Adjustment," a Tennessee Williams comedy that one embassy official said "is hardly subversive." They checked the documents of those who tried to visit the embassy, turning away Poles but allowing others to pass. It was the first time the entrance to the embassy here was barred since the early days of martial law in December 1981.

Polish officials also have protested bitterly about the U.S.-funded radio stations, Voice of America and Radio Free Europe, whose Polish-language broadcasts are blamed for encouraging Sunday's demonstrations. The Warsaw government views the broadcasts as part of an "aggressive and slanderous" American campaign to keep Poland unstable.

The dead man in Nowa Huta, site of Poland's largest industrial plant, the Lenin Steelworks, was identified as Ryszard Smagur. He was married, with a 2 1/2-year-old son, and employed by a small printing cooperative. Friends described him as never having been a Solidarity member, being largely indifferent to political activity.

The death announcement was carried inconspicuously in the accidents column on the second page of the local newspaper, Echo Krakowa. The paper said that Smagur was found unconscious in the late afternoon lying in a street where police and demonstrators had clashed. He died shortly after he was found.

A widespread rumor in the town said he had been struck in the throat by a tear gas canister or smoke grenade, but a semiofficial source in nearby Krakow said authorities were still in the process of sorting out conflicting eyewitness accounts of Smagur's injury.

He is the first person killed in street clashes in Poland since Oct. 13, when a plainclothes policeman fatally shot a young electrician, also in Nowa Huta, during a demonstration against the banning of Solidarity that month. Smagur was the 16th person the government has reported having died as a result of street violence since martial law was declared 16 1/2 months ago. Military rule was suspended in December.

The fighting in Nowa Huta, a city of about 200,000, appeared to have been among the fiercest in the 20 cities and towns that authorities said had pro-Solidarity rallies yesterday. Polish television last night broadcast brief pictures of demonstrators in Nowa Huta hurling rocks at security forces, who were reported to have dispersed the crowds with water cannons and tear gas.

As the unofficial toll of those detained during the nationwide melees rose into the hundreds, the government announced this evening that courts already have passed some sentences under summary judgments but gave no information on the number of cases involved.

Yesterday's sizable turnout, surprising in view of the massive police presence mounted against it and what had been a trend of decreasing public participation in such demonstrations, raised questions about how Polish authorities intend to handle the enormous crowds expected for the planned June visit of Polish-born Pope John Paul II. But there was no sign today that the government may be considering calling off the papal pilgrimage.

"The opposition suffered another defeat," declared Rzeczpospolita, the Warsaw paper that closely reflects the thinking of the council of ministers. The same conclusion echoed throughout the Polish press.

In contrast, Walesa, clearly enthused by yesterday's response, termed the rallies "a very successful action" but made plain he preferred talking to demonstrating.

"We will not pull the country out of its crisis with demonstrations," he told reporters after working a day shift at the Lenin Shipyard in Gdansk. "We wanted to show the authorities that we exist, that they should see our trade union and start talking with it."

He said Poland's Communist leaders should "come to their senses," adding "there is an urgent need for negotiations."

Polish officials have dismissed Walesa's pleas for new talks, arguing that Solidarity in the end became an antistate movement under a misguided, corrupt and conspiring leadership.