A singing, humming and joking Pope John Paul II today transformed a pontifical mass before a vast audience at Poland's holiest shrine into a personal celebration that was part revival meeting, part family reunion.

Speaking from an altar specially built on the ramparts of the 14th Century Jasna Gora Monastery to hundreds of thousands of worshippers in the fields below, the pope told his followers, "I am here today. I am again with all of you, dear brothers and sisters."

Time and again throughout a sermon devoted mainly to religious matters, the throng broke in with applause and song. The pope joined in a rich baritone that made up in vigor what it lacked in tone.

"Sto Lat, sto Lat," the crowd sang, "a hundred years, a hundred years, may he live a hundred years." The song is roughly the polish equivalent of "For He's a Jolly Good Fellow," a tune more appropriate to a rollicking celebration than to a solemn mass.

With each day of this pilgrimage through his native Poland, the pope seems to stray further from papal formality, exuding a relaxed and warm feeling of the traveler come home. His huge audiences, seeming to sense this and thrilled by his presence, are drawn more and more into the sermons and services with their voices and applause.

"So again I've said something that was not in the text," the pope said jokingly at one point. "They will be mad at me for lenthtening the service," he quipped, perhaps sensing chagrin on the part of the schedule-keepers.

"I will get back to the sermon, but I can't help taking these moments of joy with the people," he said. "But what can you do with a Slavic pope?" he then asked spontaneously, and the crowd laughed with him.

Then he introduced his visiting bishops from all over the world invited to witness the papal voyage, the first trip ever by a pope to the communist world.

The pattern of lighthearted banter continued throughout the day.

In a meeting tonight with young people, the pope described himself as a "sinner who likes to sing." The informality is as much a surprise as it is a delight to Poles.

Today's mass was set against the backdrop of the ancient, gray-walled monastery, festooned today with flutering Vatican and Polish flags, religious tapestries and a huge gold-and-red canopy over the papal throne. This gave it much of the color and pageantry of a medieval ceremony - but with a decidedly contemporary pope.

During his first two days of a nine-day stay in Poland, the pope had touched on some sensitive themes, such as human rights and spiritual freedom elsewhere in Eastern Europe.

But today, in what was a high point of the religious aspect of his trip, the pope kept largely to theological matters particularly the history of the Catholic Church as represented by the monastery's revered Black Madonna, an icon that for 600 years has been a symbol of Polish nationalism and religion.

"If we want to know how [Polish] history is interpreted by the heart of Poles, we must come here," he declared. "We must listen to this shrine. We must hear the echo of the life of the whole nation in the heart of its mother and queen.

"The last decades have confirmed and intensified that unity between the Polish church and its queen," he said, a clear reference to the rising power and influence of the church during the years of Communist rule here.

In the post-war era, Poles in large numbers have gathered to pray in Czestochowa (pronounced chest-uh-HO-va) on holy days, ceremonies that often brought the church's conflict with authorities into the open. The crowds and the pointed sermons by the Polish primate, Cardinal Stefan Wyszynski, frustrated the Communist Party leadership more than any religious events until now.

So important has the Black Madonna painting become in Poland, that the pope is known to be anxious to return in 1982 for the 600th anniversary of the shrine's arrival here from Hungary.

Today, for the second time since he arrived in Poland, Pope John Paul referred to the frustrated wish of the late Pope Paul VI to visit Poland.

"He was the pope that did so much for the normalization of the life of the church in Poland," John Paul said. "He was the pope of our millenium. It was for the millenium [of the Polish church in 1966] that he wanted to be here as a pilgrim." As before, Pope John Paul did not mention that the Polish government had refused to give Paul VI permission to come here.

While the size of today's crowds, like those at other stops, was impressive, reports persist that even more people wanted to come but were discouraged from doing so. Some priests from the Jasna Gora Monastery were said today to have complained about roadblocks several miles from the town center that prompted some travelers to turn back.

Officials deny there has been any interference. Nonetheless, despite the pope's visit, this was a work day in Czestochova, a steel mining town in southern Poland. No allowances were made for attendance at the papal mass.

The bickering between government authorities and the church about the visit, while not serious, is a persistent irritant in the pope's trip so far.

The church complained that Polish journalists were circulating false reports that widespread deaths had been hushed up during the frenzied reception for John Paul II when he visited Mexico in the spring. The purpose of the reports would have been to scare people from coming to the pontiff's stopovers.

Authorities, on the other hand, say the church has been slow in handing out tickets to would-be worshippers, which explains why some Poles who wanted to do so were unable to see the pope in person.

Television coverage of main events continues locally and excerpts appear on the national evening news, but cameras focus on the pope and other religious figures, almost never panning the vast crowds.

Party newspapers are carrying short, factual accounts of each day's events without commentary. The official radio simply reports that the pope has made one stop or another, without quoting what he said there.

The pope will remain in Czestochowa for two more days to attend a conference of Polish bishops and meet with people from the surrounding areas. CAPTION: Pictures 1, 2, and 3, Pope John Paul II breaks down and weeps as he is momentarily overcome with emotion during a visit to the Black Madonna shrine in Czestochowa, Poland. UPI; Picture 4, Worshippers surround the Jasna Gora Monastery for Pope John II's visit. UPI; Picture 5, The pope, in a cavalcade in Czestochowa, kisses a young girl from the crowd. UPI; Picture 6, Map, no caption, By Richard Furno - The Washington Post