Pope John Paul II may celebrate a mass on the Mall at the end of his visit to the United States this fall that could attract the largest crowd ever in Washington, U.S. Park Service and church officials said last night.

The Archdiocese of Washington has reserved the Mall for October 5-7 as possible dates for the mass, although the exact time for the pope's expected visit here has not been set.

A visit to Washington would be the first by a pontiff and the mass could attract as many as a million worshipers and observers, a U.S. Park Service spokesman estimated. Such a crowd would equal or surpass the one that watched the Bicentennial July 4 fireworks display which drew about a million people, the spokesman said.

Although plans for the pope's visit are still unsettled, church officials here said they expect the pontiff to conclude a week-long visit to the United States by saying a farewell mass here. The Vatican said the pope was expected to meet with President Carter while he was in the country, but no date or site was mentioned.

The archdiocese applied on August 1 for the Park Service permit to hold the outdoor mass on one of the three October dates, according to Park Service spokesman George Berklacy. Permission was granted immediately, he said.

Berklacy and Father Maurice Fox, a spokesman for the archdiocese, emphasized that only the initial planing has begun for the event. Fox said the church expects final confirmation of the pope's itinerary and approval for the mass to come from the Vatican sometime this week.

Pope John Paul, who became the 263rd pontiff last October, recently completed a historic pilgrimage to Poland, his homeland. He also has made an extensive journey to Mexico.

His week-long visit to this country is expected to include stops in Chicago, Baltimore and possibly Detroit, in addition to New York where he will address the United Nations General Assembly on October 2.

The Park Service and church officials here said the pope's outdoor mass may coincide with the annual National Folklife Festival. Berklacy said the archdiocese "is delighted" with the prospect and that church officials indicated they may also sponsor music and other attractions "to make the occasion festive."

The "tremendous logistics" of accommodating such a crowd will be worked out by a committee of church, Park Service and other federal, District and area officials, Berklacy said.

Church officials indicated they expect to build a stage about 50 feet deep, 100 feet long and 10 feet high for the Mall service, Berklacy said. "I think it might have to be higher," he added, "to accommodate the crowd."

Church and park service officials are considering several possible sites for the stage between the Capitol and the Washington Monument, Berklacy said.

Before coming to the United States, Pope John Paul is scheduled to visit Ireland on September 29 and 30. He will fly directly to New York from there.

An invitation to address the United Nations was extended to the pope by U.N. Secretary General Kurt Waldheim in May. Church officials have said that Pope John Paul accepted the offer in order to dramatize his support for the international organization.

Pope Paul VI who died last year, visited New York in 1967, also to make an address to the United Nations. However, he visited no other cities in this country, and confined his New York trip to the United Nations, a tour of the city and the celebration of a mass in Yankee Stadium.