Keeping alive a tradition begun by the Rev. Jerzy Popieluszko, the Polish priest slain last month, more than 20,000 Poles gathered tonight outside the white stone church, where he is buried, for a service affirming his martyrdom.

Before a vast audience sprinkled with Solidarity banners and clogging the sidewalks and streets around St. Stanislaw Kostka Church, a Warsaw pastor, Jan Sikorski, declared the continuation of Popieluszko's monthly masses for the homeland -- "a mass that was meant to be canceled."

Popieluszko won national prominence -- and drew government attacks -- by delivering strongly nationalistic, pro-Solidarity sermons on the last Sunday of every month. Three secret police officers are charged in his murder last month.

His superior, the Rev. Teofil Bogucki, preached tonight from the church's outdoor balcony, setting Popieluszko in the context of a long line of Polish clerics whose tragic deaths eventually yielded something positive for the nation.

He said Popieluszko, too, "although just a simple priest," was able to create "a turning point" through the masses whose "words of truth and encouragement" were gratefully received by many Poles "yearning to be free."

"No one knows what good will come of this," said Bogucki to thunderous applause. "His cult is growing, and vain are the attempts to defame this or create obstacles to it. This is not the way to win the Poles, gentlemen. Nothing can stop the nation marching toward its sun of freedom, solidarity and independence."

Continuation of the masses poses a dilemma for the Warsaw government. To let such blatant affronts to communist authority proceed risks the displeasure of powerful party members and the Soviets. On the other hand, to try to force an end to the services could rupture relations with Roman Catholic Church leaders who have been instrumental in maintaining peace in the country.

Cardinal Jozef Glemp, the Polish primate, leaves Monday for Rome and a meeting with Polish-born Pope John Paul II at which the killing of Popieluszko is expected to be discussed. Solidarity leader Lech Walesa said today that he had met with Glemp yesterday for an hour about a "broad scope of issues," including the future of Polish trade unions and the handling of the Popieluszko affair by the authorities.

Walesa and Glemp have helped to keep calm in Poland recently by issuing appeals for restraint after the kidnaping and killing of Popieluszko.

Tonight's service came at the end of a weekend during which both sides in the Polish struggle reaffirmed opposing programs. Leaders of the official trade unions, which replaced the Solidarity movement after it was crushed, then legally dissolved under martial law in 1982, gathered in the mining town of Bytom in southern Poland to appoint a central representative council. The action marked a major step toward a nationally coordinated union structure.

Meanwhile, underground leaders of the banned Solidarity movement urged Poles in a communique reaching western correspondents in Warsaw to continue to boycott the government-sanctioned unions, except those that publicly declare recognition of the principle of trade union pluralism. Opposition activists regard the official unions as hardly much freer of communist control than the pyramidal labor association structure that collapsed during the 1980 workers' revolt that produced Solidarity.

Delegates to the Bytom conference were quoted by the Polish press agency as saying that the new council did not represent a rebuilding of the old union organizational structure. Some unions had spoken before the meeting of the need to coordinate the bargaining positions of the roughly 20,000 local factory unions and their national associations in order to confront the government in wage negotiations.

Speaking for the government at the union meeting, Deputy Premier Mieczyslaw Rakowski reasserted official support for the unpopular unions. In what appeared to be a rejection of the opposition's demand for a system of competitive unions -- in place of the one-per-factory rule now in force -- Rakowski stressed the importance of sustaining a "uniform" trade union movement.

"Not all people understand this today," he remarked, referring to the fact that the current unions can claim less than half of Solidarity's former membership of 10 million. "They are still keeping aloof."

The Solidarity statement, dated Nov. 19 and signed by the five regional members of the underground Temporary Coordinating Commission, instructed Solidarity cells in factories to press for pay increases and organize opposition to the longer working hours due to take effect in January.