Pilgrims waving Polish, Czech or Bavarian flags and others of all ages and races attended the morning mass by the pope in front of the pink and white baroque church.
Mariazell, a site of pilgrimage for 850 years, "symbolizes an openness which... transcends physical and national frontiers," the pontiff said after arriving in Vienna on Friday for a three-day visit to the predominantly Roman Catholic nation.
The site at a mountain some 110 kilometres (70 miles) southwest of Vienna is visited by about one million pilgrims every year from Austria and nearby countries such as Hungary, the Czech Republic, Slovakia, Slovenia, Poland, Croatia and Bosnia-Hercegovina.
Many had traveled by bus. Others came by train, but a few walked with rucksacks on their backs, up the mountain to Mariazell, observing their pilgrimage in the traditional way.
American Mary Jo Szekeres, 22, said she had traveled nine hours Friday from Piestany in Slovakia where she works as an English teacher, to attend the mass.
The trip was well worth it, she said. "I feel a more personal connection to the pope now. I've seen him, I've heard him speak."
Some 30,000 people turned up Saturday, waving commemorative blue scarves and chanting "Benedetto, Benedetto."
Among them were a group of Italians from Udine, a couple of Nigerian priests, Chinese Roman Catholics from Vienna, a Filipino family and two young Frenchmen, who had traveled specially from Lyon and were trying to argue their way past security for a closer look at the pontiff.
The mood in front of the basilica was subdued before the pope arrival due to the bad weather.
Patches of snow covered the sides of the road to Mariazell Saturday morning, and non-stop rain only seemed to increase as the pope approached.
The inclement weather had even forced the pontiff to travel to Mariazell by car instead of by helicopter as initially planned.
But a loudspeaker announcement that "the pope is in Mariazell" drew cries from the faithful flocked at the square, and those who had sought shelter under storefronts rushed to the side of the road as the first police cars announced the arrival of the Popemobile.
The first pilgrims had started arriving in Mariazell at 6:00 am (0400GMT).
The lucky ones were seated on bleachers around the square. Others were herded into pens in front of giant screens set up around the small town.
A nun, Sister Angelena, from Bruck an der Mur in southern Austria, had come to Mariazell in 1983 for a visit by the late Pope John Paul II's visit in 1983.
She was out in front of the basilica early on Saturday.
For her, the Pope's visit showed that "the Church lives, even in times of trouble. The Church will not founder. God's spirit is still strong nowadays."
Youngsters from all nine archdioceses in Austria were also present, wearing warm robes under their ponchos.
For 16-year-old Victoria Wallner from the Vienna archdiocese, the pope's visit had a spiritual as well as a practical purpose.
"We can be closer to God and Jesus Christ... and get to meet new people," she said.
She joked that her prayers for sunshine had not been heard.
"Instead we'll shine from our hearts," her brother Nicolaus replied.
American theology student Erika Olson, 20, marveled that the pope was visiting Austria just as she was here for a semester.
"When I go back and read his writings, I'll hear his voice from now on," she said.
"I can't feel my feet," she added however with a laugh after standing for hours in the cold.
Faithful or not, many said they just had to make this pilgrimage.
Susanne Laubenbacher from Graz in southern Austria said she was not a strong believer and found the pope's language too harsh.
But she added: "I would not have wanted to miss this."
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