Pope Paul VI, eulogized as a "true prince of peace," was buried last night in St. Peter's Basilica after an unprecedented outdoor funeral mass celebrated by the world's cardinals.

The late pontiff, who for 15 years led christendom's largest church through change and conflict before he died last Sunday, was interred as he wished, in a simple grave in the basilica's grottoes, not more than a hundred feet from that of his predecessor, Pope John XXIII.

More than 75,000 persons were present in St. Peter's Square as 104 scarlet-robed cardinals sang prayers in unison in the concelebration of the solemn requiem mass. Tens of millions throughout the world watched the live television broadcast of the rites.

It was the first time that so many members of the Sacred College of Cardinals participated in last rites for a pope and the outdoor ceremony broke with Vatican tradition of conducting papal funerals inside St. Peter's.

At the end of the two-hour funeral, the mourners broke out in respecful applause as the plain wooden coffin was carried through the massive bronze portal into the basilica.

It was the only outward show of collective emotion the crowd displayed, and it seemed to underscore the contrast between Pope Paul's frail and quiet bearing and the extroversion of his predecessor, affectionately known by Italians as il papa simpatico.

Attending the funeral were 105 delegations of officials from foreign countries, more than double the number that attended Pope John's funeral in 1963. The number was attributed to Pope Paul's intensive efforts to establish Vatican ties to the Third World and to Eastern Europe nations.

Foreign dignitaries included U.N. Secretary General Kurt Waldheim, Zambian President Kenneth Kaunda, Spanish Premier Adolfo Suarez and a dozen other prime ministers and vice presidents, as well as officials from some Communist countries. President Carter's wife, Rosalynn, headed the U.S. delegation, which included Sen. Edward M. Kennedy (D-Mass.) and New York Gov. Hugh Carey.

The foreign delegations provided a striking contrast to the thousands of ordinary admirers of Pope Paul who, casually attired, strolled through the cobblestoned piazza in front of St. Peter's and listened half-attentively to the lengthy liturgy.

Under clear blue skies and a warm August sun, many of the Romans and foreign tourists mingled along the stone steps of the colonnade, eating sandwiches and fruit.

But immediately in front of the magnificently adorned portico of the basilica, the cardinals and other prelates of the church held the attention of those mourners within viewing distance as they went through the ancient rituals celebrating death and spiritual rebirth.

As the Sistine choir sang prayers, Pope Paul's coffin was placed on a rectangular oriental carpet in front of a 10-foot-high candle, and the cardinals, dressed in scarlet vestments and wearing white miters kissed the altar.

Cardinal Carlo Confalonieri, dean of the sacred college, praised Pope Paul in a homily for bringing "one surprise after another to the various continents" that he traveled during his pontificate. Pope Paul, far more than any other pontiff, journeyed abroad as an emissary of Catholicism.

Pope Paul, said Confalonieri, was "a great spirit, of keen intelligence . . . a voice speaking out in defense of truth and justice, condemning violence in every form . . ."

"History will remember his name for many reasons . . . He could say with the apostle, 'be imitators of me, as I am of Christ,'" Confalonieri said.

More than 7,000 policemen, antiterrorist agents and sharpshooters were assigned to the Vatican City to protect the foreign delegations during the funeral, but for the most part they could not be seen by the mourners because many of them blended into the crowd and others were held in reserve blocks away from the square.

Helicopters circled overhead a discreet distance away, providing a reminder of the need for security at public events in Italy's capital, but the funeral went on without incident. Police and khaki-uniformed carabinieri strolled nonchalantly around the square, talking with tourists and showing little evidence of concern of violence.

The walls of St. Peter's Square were plastered before the funeral with posters placed by a conservative Roman Catholic organization, Civilta Cristiana, calling on the cardinals to elect a strict doctrinaire as pope. The organization had frequently criticized Pope Paul and the revisions of the Second Vatican Council as too liberal.

The conclave to elect a successor to Pope Paul will begin Aug. 25, when the entrances to the Vatican quarters of the cardinals will be bricked shut, the windows covered with opaque paint and the deliberations held in extraordinary secrecy.

In the Sistine Chapel, where the voting will take place, canopies have been suspended over the cardinals' chairs. At the end of the balloting all will be lowered except that of the new pope.

There was significant Communist representation at yesterday's funeral - more so than at any other pontiff's last rites - largely as a result of Pope Paul's active policy of church overtures to Eastern European governments.

Hungry sent Vice President Rezo Trautman, in a gesture viewed here as an effort to better relations with the Vatican. Last year, Communist Party chief Janos Kadar visited Pope Paul in what then was seen as a signal of conciliation in sharp contrast to the animosity that existed during cardinal Jozsef Mindszently's 15-year refuge at the U.S. embassy in Budapest.

Poland's Vice President Zdzislaw Tomal also attended the funeral, along with Poland's Roman Catholic primate, Cardinal Stefan Wyszinski.

Catholic prelates from five Eastern European nations attended the requiem mass, led by a Polish delegation that represents an 88 percent Catholic population in that country.

Also represented yesterday was Czechoslavakia, whose 8 million Catholics comprise 60 percent of the population.

While Italian newspapers of all political persuasion have been devoting heavy coverage to the death of the Pope, there have been unexpectedly fervent tributes from the Communist publications.

Throughout Rome, the Italian Communist Party has hung posters declaring, "Communists weep for the death of Paul VI." Visitors to the papal summer palace at Castel Gandolfo to pay homage to the pontiff included Pietro Ingrao, Communist president of the lower house of parliament, and Rome's Mayor Carlo Giullo Argan, a Marxist who has the backing of the communists.

Italian Communist Party leader Enrico Berlinguer said of Pope Paul, "We Communists . . . appreciated the positions developed developed and the acts made under his pontificate toward promoting dialogue . . . among peoples and states of different faith, ideals and social regimes."

Flags at Communist Party headquarters throughout Rome were flown at half staff and were adorned with black ribbons yesterday. Headlines in Communist newspapers have been bordered in black.

The Communist newspaper I'Unita headlined a full-page obituary, "Constant activity for peace and a different dialogue with the world."