WARSAW, Poland (AP) _ Solidarity leader Lech Walesa told supporters Tuesday he won't press for completely free elections during talks with authorities because he first wants them to restore his union's legal status.

A spokesman for Solidarity said the union wants open parliamentary elections eventually, but is willing to delay fulfillment of that goal until it obtains its prime objective of legalizing Solidarity. The Communist Party currently controls nominations and ensures itself a majority of seats.

In another development, government spokesman Jerzy Urban sought to play down overly optimistic expectations for the talks that began Monday, known as the ''round table.''

''One should not exaggerate the hopes evoked by the round table,'' he said. ''It is an event of significance ... but one should not imagine that the round table shall solve the problems standing before Poland, that it is some magic wand.''

At the formal talks with 57 participants, including a 25-member opposition group led by Walesa, authorities offered Solidarity legalization if the union endorses a package of economic and political reforms.

The authorities also said Solidarity must agree to take part in ''non- confrontational'' elections this spring that would give the opposition some seats in parliament but still guarantee communist control.

Three working groups dealing with specific elements of the proposed compromise - the economy, political system and trade unions - begin meeting Wednesday. The negotiations are expected to last four to six weeks.

The proposal on entering the political system is most sensitive for Solidarity, which does not want to be in a position of endorsing an electoral system in which the results are determined beforehand by the Communist Party.

However, both Walesa and Solidarity national spokesman Janusz Onyszkiewicz indicated the union is ready to defer demands for free elections in order to win legalization now.

''I publicly answer with full responsibility ... that in these conditions (I) will not seek a confrontation on the subject of elections,'' Walesa told a group of 500 workers and supporters at the Technical Building Institute in Warsaw.

''The pluralism which we will obtain now may create such a possibility one day,'' he said, but now ''we are not ready for that.''

Walesa's was welcomed warmly by the institute's director and cheered by more than 500 supporters and workers. He spent a hour answering questions and defending his decision to engage in talks with authorities.

''The road to the table was difficult. Many people think it doesn't make sense. Many say bluntly: 'Lech, you believed in 1980. You were naive. And now you are still naive.' My answer is: 'I didn't believe in 1980 and I don't believe now,'' he said.

''But there is one Poland ... I am doing everything to take advantage of all chances to build agreement, to build a better Poland,'' he said.

If the talks yield ''pluralism of associations'' - the right for various groups to form associations and clubs - it will give the opposition a chance to ''prepare people, the wise and reasonable ones, prepare them for better elections'' in the future, Walesa said.

Onyszkiewicz, who held his own news conference at Warsaw University for about 200 Polish and foreign journalists, said Solidarity would not be nominating a list of candidates to future elections. But he said some Solidarity members could run as individuals, not as representatives of the union.

Asked about the suggestion made Monday by Interior Minister Gen. Czeslaw kiszczak that the elections be ''non-confrontational,'' Onyszkiewicz said the union understood Kiszczak to mean non-divisive, ''not an opportunity for sharpening of conflicts, for an open war.''

Onyszkiewicz said Solidarity's ''priorities are obvious. We have been for free elections for a long time, since 1981. But we allow a certain temporary time, not too long, when those free elections can be partial to some extent.''

Government sources have said the authorities are willing to offer the opposition 20 percent to 40 percent of the seats in parliament after elections to be held in April or May.

Polish elections are structured so that people of different parties never oppose each other. The party controls who is nominated, ensuring that the Polish United Workers (Communist) Party always wins a majority of seats.