For the first time in American church history, a Roman Catholic pope will meet and pray with a major gathering of leaders of other Christian denominations when Pope John Paul II visits Washington on Oct. 7.

Between 300 and 500 top leaders of Christian churches in this country -- mainline Protestant, evangelical, Episcopal and Eastern Orthodox -- will receive invitations shortly to the extraordinary gathering at the Catholic University of America.

The invitations will be addressed to "heads of denominations or their representatives," said the Rev. Peter Sheehan, an ecumenical officer of the National Conference of Catholic Bishops, which is helping to coordinate the papal trip.

During his week-long tour of this country, the pope also is expected to meet with American Jewish leaders, probably in New York City, church officials said.

Until the Second Vatican Council that concluded 14 years ago, Catholic doctrine precluded formal contact with other Christian bodies.

In the early years of his pontificate, Pope Paul VI met with some heads of other Christian communions and also journeyed to the headquarters of the Protestant and Eastern Orthodox World Council of Churches in Geneva, Switzerland.

The planned ecumenical meeting here marks the first time that such a broad spectrum of Christian leaders has been brought together to meet with the supreme pontiff of the Roman Catholic Church.

"I guess it could only happen in the United States, because of the religious diversity here," said Sheehan.

Bishop Thomas C. Kelly, general secretary of the National Conference of Catholic Bishops, said the meeting is expected to run 30 to 45 minutes and will include "some time given over to prayer together" -- something that could not have happened as recently as 15 years ago.

The pope also is expected to address the American Christian leaders.

In the years since the Second Vatican Council pointed the way to friendlier Catholic relationship with other Christian communions, a series of continuing dialogues between Catholics and other major Christian traditions has been under way.

The dialogues, conducted at what is to most lay members arcane theological levels, have had little visibility to the public at large. But in implementing the Vatican II mandate to press for true Christian unity and understanding, they have helped pave the way for meetings such as the one planned here.

Going from virtually no contact a couple of decades ago, interfaith relations in this country have progressed to the point where the heads of three major faith groups -- the Catholic Bishops Conference, the National Council of Churches, and the Synagogue Council of America -- meet regularly here or in New York City to discuss mutual problems and concerns.

In fact, Dr. Claire Randall, general secretary of the Protestant and Orthodox National Council of Churches, was asked by Catholic officials to draw up the list of church leaders who should be invited to the meeting here. The list includes both NCC-member churches and groups, such as the Southern Baptists and other evangelicals, which have declined to join the liberally oriented National Council.

Pope John Paul's meeting with American leaders of non-Catholic churches will come on the final day of his busy week-long visit to this country. It will be squeezed into a packed Sunday schedule which includes a service for nuns of the Washington area at the Shrine of the Immaculate Conception, a meeting with heads of American Catholic institutions of higher learning and representatives of learned societies, and a public mass, to be celebrated on the Mall at 3 p.m.

A number of religious leaders -- Catholic, Protestant and Jewish -- also have been included in the White House guest list for the reception which President and Mrs. Carter are giving for Pope John Paul II on October 6.