Pope John Paul II stepped up his pressure on Poland's military government today, insisting that the only solution to the Polish crisis is the full restoration of the country's independent trade union, Solidarity.

In one of the Polish-born pontiff's firmest statements to date regarding developments in his native land, the pope emphasized that "the restoration of an effective and complete respect for the rights of working men, and especially their right to a union which has already been established and legalized, is the only way out of this difficult situation."

Speaking at the Vatican to 30 trade union leaders, including six Solidarity members in exile, the pope said the harsh measures applied by the Polish government "cannot force us to forget that this union acquired, and still possesses, the character of an authentic workers' organization, recognized and confirmed by the organs of the state."

Solidarity, the pope said, "is and remains an autonomous and independent union, faithful to its initial aspirations, rejecting violence even in the difficult situation it faces today, and still hoping to be a constructive force for the nation."

The pope's unequivocal statement was a direct challenge to the military regime of Gen. Wojciech Jaruzelski, which banned Solidarity in imposing martial law on Poland Dec. 13. The pope's remarks came in the midst of a week of intense private discussions between Vatican officials and a three-man ecclesiastical delegation from Poland headed by the Polish primate, Archbishop Jozef Glemp.

Today's statement by the pope was in marked contrast to the conciliatory language used by Glemp at a mass in Rome last Sunday. The Polish archbishop stressed that like the church, Solidarity will have a place in Poland's future, but he emphasized above all the need for peaceful dialogue with the country's military rulers.

The difference in tone may have partially reflected the fact that the archbishop, who returns to Warsaw Thursday, was speaking outside his country and at a time when Polish authorities have hinted that they may soon be willing to meet with Solidarity officials to discuss future labor organization rules.

But Vatican sources said it also indicated a rift between Glemp and hard-liners in the Polish church leadership.

"It's hard to decide on a strategy when you want to use your leverage without, however, either compromising yourself or damaging the chances for dialogue," said one long-time Vatican analyst, who said the rift should not be blown out of proportion.

The Vatican has been extremely close-mouthed about the substance of the talks between the pope and the archbishop, who arrived here last Thursday along with Cardinal Franciszek Marcharski, the pope's successor as archbishop of Krakow, and archbishop Henryk Gulboniwicz from the western diocese of Wroclaw. Vatican sources say a major purpose of Glemp's visit--his first since the military takeover--was to discuss how the church should deal with the ongoing crisis.

The pope has sought to avoid confusion between what he sees as his moral role and political activism. But his awareness of the church's influence in Poland, which is overwhelmingly Catholic, and mounting concern over mass arrests and state-imposed loyalty oaths have led him to gradually sharpen the tone of his criticism.

Ever since the imposition of martial law, both the Polish church and the pope have made it clear that avoidance of further bloodshed is a top priority.

Early in the crisis, church authorities clung to the hope that a compromise could be hammered out that would restore at least partial freedoms to Solidarity in return for assurances to Moscow that the union movement would restrain its demands. That hope was fed in part by the government's apparent eagerness to have Polish church leaders play a mediating role.

But eight weeks later, much of the church's initial optimism has waned and it is clear that the church has failed to win any substantial concessions from Poland's military rulers.

In early January, Glemp reversed an earlier stance and agreed to meet with Jaruzelski despite the fact that neither Lech Walesa nor any other Solidarity leaders was present. But the meeting, held Jan. 9, proved unproductive.

Vatican insiders say that the pope is deeply concerned about the stalemate in Poland and in particular about the the military government's success in imposing a crackdown. As one sign of the Vatican's uncertainty about how to deal with the crisis, neither the pope nor the archbishop has made any references to the pontiff's invitation, extended before martial law was imposed, to visit Poland this August for the 600th anniversary of the country's holiest shrine at the Jasna Gora monastery in Czestochowa.

The pope is under growing criticism here for constantly referring to Poland while failing to speak out on events in El Salvador, scene of a bloody civil war. But Vatican observers have little doubt that the pope's frequent comments on developments in his homeland will continue.

"He knows what effect his election and his June 1979 visit to Poland had on events there, and he appears to feel not only involved but responsible," said an Italian clergyman.