Sydney - Catholic youngsters from 170 countries, gathering in Sydney to meet the pope and get to know each other, were urged Tuesday to not only hold fast to their faith but also help holding back the tide of secularism.

"Don't spend your life sitting on the fence, keeping your options open, because only commitments bring fulfillment," Cardinal George Pell, leader of Australia's 5 million Catholics, told a congregation of up to 150,000 on the foreshore of Sydney Harbour for the opening mass of the week-long World Youth Day celebrations.

"Happiness comes from meeting our obligations, doing our duty, especially in small matters and regularly, so we can rise to meet the harder challenges," Pell, the archbishop of Sydney, said in his homily.

Some 3,500 priests, 400 bishops and 26 cardinals were on hand to lead what organizers said was the "biggest and grandest mass ever celebrated in Australia."

It was certainly the most spectacular. Kind weather on a crisp winter day provided a marvelous tableau of youngsters draped in national flags with the water of the harbour before them and a cloudless sky above.

World Youth Day, held somewhere in the world every three years, is an enormous magnet for young believers. Around 125,000 have arrived from abroad to join 100,000 young Australian Catholics. They are ensconced in warm Sydney homes, camped school classrooms or sleep on concrete floors in sports stadiums way out of the city.

Karol Darmet, a 24-year-old marketing executive from Grenoble, France, said being at the Sydney jamboree had bolstered his faith and brought him new chums from around the world.

"It's great to be among so many other young Catholics," he said. "It makes us young people feel belonging, that we are together and that there's nothing wrong with our faith, with what we believe."

Pope Benedict XVI flew to Australia on Sunday and is resting for three days before making a grand water-borne entrance on Sydney Harbour. On his first visit to Australia he has promised to make a formal apology to parishioners sexually abused by wayward clergy.

With the famous bridge and Opera House as the backdrop to the papal flotilla, organizers expect a picture-perfect moment on Thursday - one that pilgrims will treasure for ever and that television stations will beam around the world.

The highlight is expected for Sunday when the 81-year-old pontiff presides over an outdoor mass for up to 500,000 of his flock.

Pilgrims will trek to the racecourse venue over the whole of Saturday and spend a night in sleeping bags under the stars awaiting the dawn and the arrival of the Holy Father.

World Youth Day spokesman Bishop Anthony Fisher said the infusion of teen spirit and unbridled joy had wowed the often grumpy residents of Australia's biggest city. He said pilgrims had won over those who had complained of the expense and of the disruption.

"Even people who have been a bit cranky with World Youth Day, or have their own other issues whatever they are, will be swept along by the beauty and joy of these young people, by their goodness, and they'll just want to be part of that," he said. "So I suspect we won't have much in the way of protests."

Gay and lesbian activists have pledged to hand out condoms and march on venues to protest the church's teachings against homosexuality, enforcing the celibacy of the clergy and restricting the role of women. (dpa)