Several hundred pilgrims assembled here today to form the advance guard of the hundreds of thousands of faithful supporters expected Friday for Pope John Paul II's open-air mass at the nation's most sacred shrine.

The pope's stop at the Basilica of Our Lady of Lujan is expected to be the high point of his 32-hour visit to Argentina, which was rapidly arranged to allow the pope to make his historic May visit to Britain even though it was fighting Argentine troops for control of the Falkland Islands. Argentina is predominately a Roman Catholic country.

The early enthusiasts here, mostly adolescents, had staked out their sleeping areas here by this afternoon, as frenzied preparations for the papal mass continued inside the pink stone, neo-Gothic basilica. Hundreds of thousands of faithful are expected to crush into Lujan's central mall in front of the basilica to hear the pope.

The leader of the Roman Catholic Church will be ferried about 43 miles by helicopter from Buenos Aires following his visit with the president, Gen. Leopoldo Galtieri, and other members of the junta and Cabinet. Although the pope is keenly aware that politicians might try to capitalize politically on his visit, he may not be prepared for the spectacle being readied for him by the various, often warring, political organizations here.

"The pope and Peron are as a single heart" reads one sign planned for the mass here. The pope's visit will provide the successors of the late populist leader, Gen. Juan Peron, with their best opportunity for a massive public rally since before the overthrow of his widow, Isabel, in 1976.

The junta was upset in May when the pope did not cancel his scheduled visit to Britain--the first by a pope since Henry VIII broke relations with the church of Rome--as a protest against the war over the Falklands. John Paul's subsequent offer to visit Argentina as well was initially poorly received. But the three-man junta is now expected to take advantage of every occasion to be seen in public with the pope.

Catholicism is the state religion here, guaranteeing officials contact with the pope. Galtieri will meet John Paul at the airport Friday, address the nation jointly with him during the reception ceremony, meet him again at the national palace, attend an open-air mass in downtown Buenos Aires Saturday and see him off at the airport Saturday afternoon.

The actual host of the papal visit--the Argentine Catholic Church--has been somewhat overshadowed in the turbulent events of the Falkland crisis and the domestic political breakdown.

But the church is itself split--to a greater degree than in other Latin American countries--between a traditionalist hierarchy and a change-oriented clergy. Both sides are expected to do some special pleading during a meeting with the pope Friday at noon. The progressives will be pleased if the pope publicly advocates human rights, a controversial subject in this country where thousands of people have disappeared during the military rule.

However, in the church as in all sectors of Argentine society, nationalism is the unifying factor. The progressive bishop of Santa Fe, Msgr. Vicente Zazpe, today joined the national chorus in a news editorial. He denounced Western leadership, pointedly mentioning British Prime Minister Margaret Thatcher and President Reagan by name, as "mediocre" and "short of stature."

In expectation of the papal visit, the government, the Peronists and an alliance of opposition parties each scheduled rallies for this afternoon. The meetings were supposed to express devotion for the pope, but support for the Argentine military's efforts in the South Atlantic was inevitably intertwined.

Security is tight for the pope's visit. His activities will include an open air mass in Palermo Park in Buenos Aires Saturday and a reunion with the bishops of Latin America. There will be numerous bishops present at Saturday's open-air mass, including 10 from other Latin American countries.

This is the pope's third visit to Latin America. He toured the Dominican Republic and Mexico in 1979 and visited Brazil in 1980.