BARANGAROO, Australia, July 15 (UPI) -- An estimated 150,000 young Roman Catholics gathered Tuesday near Sydney to attend the opening mass and ceremonies for World Youth Day, church officials say.

It was the formal beginning of six days of celebrations which will culminate on Sunday with a Mass celebrated by Pope Benedict XVI before a predicted 500,000 worshipers.

The scene of the celebration was a beach-side suburb of Barangaroo, a former industrial harbor that had been transformed into a festive place of worship as thousands of pilgrims chanted, sang and danced to kick off the six-day festival, the Melbourne Herald Sun reported.

Australian Prime Minister Kevin Rudd took the stage at the start of the service, presided over by Sydney Archbishop Cardinal George Pell, to welcome the visitors in English, German, Italian and French.

"G'day, and have a great time Down Under," Rudd said. "Australia welcomes the youth of the world to Sydney. Australia welcomes the youth of the world to this celebration of faith and this celebration of life."

Pope Benedict XVI, speaking in a video message to World Youth Day pilgrims shortly before the start of the event, said he looked forward to the World Youth Day events as a chance for "prayer and reflection with young people from around the world." Benedict called Australia the great "southern land of the Holy Spirit."

Like many mainstream Christian denominations, Roman Catholicism is struggling to maintain its following.

Cardinal George Pell, the archbishop of Sydney, urged the pilgrims to keep and spread the faith through self-discipline and prayer in a homily before thousands. He also referred to the distance they had traveled.

"Many of you have traveled such a long way that you may believe that you have arrived, indeed at the ends of the Earth," Pell said, the sun setting behind him. "If so, that's good, for our Lord told his first Apostles that they would be his witnesses in Jerusalem and to the ends of the Earth."

Nearly 250,000 people have registered for World Youth Day, more than half of them from overseas. Thousands of young people were staying in churches and school houses or in volunteers' homes, and were visible throughout the city, hoisting their official yellow, red and orange backpacks.

The six-day celebration began at midnight, when a giant countdown clock ticked over to read "G'Day Pilgrims" _ Good Day, Pilgrims _ drawing wild cheers from the people who gathered at St. Mary's Cathedral.

Registered pilgrims received the first of daily inspirational text messages from the pope: "Young friend, God and his people expect much from u because u have within you the Fathers supreme gift: the Spirit of Jesus - BXVI."

Every evening during the event, a light show of 20 images of the pope and the Australian outback will be projected on a pylon of Sydney's Harbour Bridge in celebration of World Youth Day.

On Tuesday, pilgrims scarfed down traditional Australian meat pies and mingled along the waterfront. A group of French pilgrims wore stuffed roosters on their heads, which they said were a symbol of France. Nearby, a group of parishioners from Nottingham, England, sported green felt Robin Hood hats.

"You see so many nationalities and you realize the church is not just Nottingham. It's a world Church," said Father David Cain, who traveled to Sydney with 20 members of the Nottingham diocese.

Ronny Guenker, a 32-year-old German pilgrim, said it was a good sign that the pope was visiting the youth festival "because we are the church of the future."

Benedict has raised expectations that he will apologize directly to victims of past clergy sexual abuse while he is in Australia, telling reporters he will do everything possible to achieve "healing and reconciliation with the victims." Activists in Australia have demanded the pontiff make a formal apology.

Benedict also signaled he will discuss the need to face up to the "great challenge" of caring for the environment, noting that global warming is an issue worrying many young people.

Young people are at the center of the battle.

"Many young people today lack hope," the pope said last week. "They are perplexed by the questions that present themselves ever more urgently in a confusing world, and they are often uncertain which way to turn for answers."

"It is my firm belief that young people are called to be instruments of that renewal, communicating to their peers the joy they have experienced through knowing and following Christ," he said.

The 81-year-old pope arrived in Australia on Sunday, but will not formally join the celebrations until Thursday.

For days, Sydney has been crowded with pilgrims from all over the world, moving around in happy groups, identifiable by the yellow, orange and red backpacks that they have been given.

It has been a formidable logistics operation. Thousands of young pilgrims are sleeping in sports halls, churches and schools across the city.

And the event has found some unusual supporters - 282 pilgrims from Argentina, Brazil, and the United States are sleeping at the Malek Fahd Islamic School in a suburb of Sydney.

"This was a good opportunity to extend our hand in friendship and break down the barriers and misunderstandings between religions," said Pinad El-Ahmad, who is in charge of inter-religious activities at the school.

"Hospitality is part of our Islamic teaching. We know that the Prophet opened his house and mosque to non-Muslims, and so it is only right that we should do the same," she said.

Cardinal George Pell, the archbishop of Sydney, urged the pilgrims to keep and spread the faith through self-discipline and prayer in a homily before thousands. He also referred to the distance they had traveled.

The six-day celebration began at midnight, when a giant countdown clock ticked over to read "G'Day Pilgrims" _ Good Day, Pilgrims _ drawing wild cheers from the people who gathered at St. Mary's Cathedral.

Registered pilgrims received the first of daily inspirational text messages from the pope: "Young friend, God and his people expect much from u because u have within you the Fathers supreme gift: the Spirit of Jesus - BXVI."

Every evening during the event, a light show of 20 images of the pope and the Australian outback will be projected on a pylon of Sydney's Harbour Bridge in celebration of World Youth Day.

On Tuesday, pilgrims scarfed down traditional Australian meat pies and mingled along the waterfront. A group of French pilgrims wore stuffed roosters on their heads, which they said were a symbol of France. Nearby, a group of parishioners from Nottingham, England, sported green felt Robin Hood hats.

"You see so many nationalities and you realize the church is not just Nottingham. It's a world Church," said Father David Cain, who traveled to Sydney with 20 members of the Nottingham diocese.

Ronny Guenker, a 32-year-old German pilgrim, said it was a good sign that the pope was visiting the youth festival "because we are the church of the future."

Benedict also signaled he will discuss the need to face up to the "great challenge" of caring for the environment, noting that global warming is an issue worrying many young people.