Thousands of pilgrims sang and danced their way through Sydney to the World Youth Day opening mass at Darling Harbour.
Central Sydney streets were filled with Catholic pilgrims, greeting each other and chanting, as they made their way down to the foreshore site at Barangaroo, which was bathed in winter sun.
Organisers were expecting more than 150,000 pilgrims to attend Tuesday afternoon's service, the first major event on the six-day World Youth Day calendar.
Between 3,000 and 4,000 priests, 400 bishops and 26 cardinals were to attend what organisers said would be the "biggest and grandest mass ever celebrated in Australia".
The sound of bongo drums and guitars was to give way to quiet contemplation and prayer when Sydney Archbishop Cardinal George Pell began the mass at 4.30pm (AEST).
World Youth Day organisers are confident almost a quarter of a million Catholic pilgrims will take part in the six-day event.
The opening mass in Sydney will be the largest ever celebrated in Australia, spokesman Bishop Anthony Fisher says.
Bishop Fisher said he expected 26 cardinals, 400 bishops and 3,000 to 4,000 priests "as well as the youth of the world" to attend.
"That will make it the biggest mass we've ever celebrated in Australia, the grandest mass in Australia," he said.
"That's more cardinals and bishops and priests even than at the previous papal masses (in Australia).
"It will be a spectacular mass, it will be a reverent mass, it will be a beautiful mass."
Bishop Fisher said the number of people attending Tuesday's mass would only be topped by the final papal mass at Randwick Racecourse on Sunday.
Organisers have said up to half a million people are expected for the papal mass.
Bishop Fisher said Pope Benedict XVI was recovering well at a retreat in northwestern Sydney residence at Kenthurst after arriving on a long haul flight from Italy on Sunday afternoon.
"I was very impressed by his energy as we went walking in the garden (on Monday) and we were having trouble keeping up with him, and he's 81, but very fit," Bishop Fisher told reporters.
"Then at the (lunch) table he was asking all sorts of questions about Australia, and about the week of World Youth Day and it really impressed me how knowledgeable he is about Australia."
He said Sydney Archbishop Cardinal George Pell was excited about celebrating the biggest mass of his life on Tuesday and would deliver an evocative homily from the Old Testament about the "valley of dry bones", which was pertinent to Australia as it battled with drought.
"The homily will be very much about hope for young people."
Meanwhile, thousands of young Catholic pilgrims from around the globe have converged on Sydney's St Mary's Cathedral.
In scenes resembling an Olympic Games opening ceremony, groups of young people carried their national flags and chanted and sang Catholic and patriotic songs outside the cathedral.
St Mary's is the site of an indoor exhibition featuring the body of Blessed Pier Giorgio Frassati, the inspirational Italian who became a role model for young Catholics before he died in 1925 aged 24.
His remains were brought to Australia from Italy especially for WYD.
Over 1,000 people lined the forecourt of the cathedral on Tuesday morning, waiting patiently and happily to be ushered inside to view the exhibition.
Once inside, the pilgrims' mood changed to one of awe and reverence.
Young Catholics also knelt and prayed before the Australian painting of the Blessed Mary and Baby Jesus, commissioned especially for Australia by the Archbishop of Sydney, Cardinal George Pell.
Rhonda Visaya and 14 others made the journey from their parish in Maui, Hawaii, and have been surprised at the beauty and hospitality of Sydney.
"It's cold, but very beautiful, and the cathedral, it's beautiful," Ms Visaya told AAP.
"I find Pier Giorgio very interesting.
"First of all, that his feast day is the 4th of July, and he wanted to give to other people."
"We need to learn more about him."
Chants of "Aussie, Aussie, Aussie, Oi, Oi, Oi" erupted several times in the cathedral forecourt.
Melbourne couple Shane and Abigail Karayi found the exhibits touching.
"You feel the energy, you feel the spirit, you can sense someone's around - you're not alone," Mr Karayi said.
Also among the pilgrims were three teenage girls from Austria who have already visited Ballina on the NSW north coast, Cairns, Uluru and Alice Springs.
"In Austria, it's much colder in winter, but it was 35 degrees when we left," Valerie Walderdorff said.
"I like to celebrate the (Catholic) mass with all the nations," Ms Walderdorff said.
"In Ballina, we were six nations together."
There were smiles and happy greetings as pilgrims from various nations waited to enter the cathedral.
Some groups were led by musicians, playing guitars and drums as they waited in line.
Others sat on the cathedral steps posing for photos.
Hyde Park was being used on Tuesday as a meeting area before groups make the short walk to the cathedral.
Earlier police reported that a Sydney war memorial had been sprayed with graffiti supporting the Pope, as well as other messages.
Police are investigating the attack on the Hyde Park War Memorial between 11.30pm and midnight (AEST) Tuesday.
Three slogans were painted on the eastern side of the memorial, including "Ratzinger rules", apparently referring to the Pope's birth name of Josef Ratzinger.
NSW Police Commissioner Andrew Scipione said the other slogans were: "God bless you diggers" and "Aussie Pommies Seppo's all allies in deed".
"This is a very serious matter," Mr Scipione told reporters.
"Whilst it might not have been derogatory in terms of the words used, we would say this is enormously disrespectful, particularly to our returned servicemen.
"As well, to a visiting head of state in the Pope."
Officers were reviewing CCTV footage and will pursue the offenders if they can be identified.
RSL NSW president Don Rowe said despite the nature of the sentiments expressed, defacing a war memorial was always unacceptable.
"People seeing that as an excuse to write on it is extremely disappointing, and it's an insult to the sacrifice which our men and women have made," Mr Rowe told AAP.
"The war memorial should never be used as a signpost for graffiti at all, so it's quite alarming to see that happen."
Mr Scipione said the graffiti had been cleaned from the memorial and no other damage was done.
Meanwhile, two Sydney activists have won a Federal Court challenge to special WYD laws carrying $5,500 fines for annoying pilgrims.
NoToPope Coalition members Rachel Evans and Amber Pike took the NSW government to court over the police powers granted for the month of July, seeking a declaration that they were unconstitutional.
Justices Catherine Branson, Robert French and Margaret Stone on Tuesday ruled the specific clause relating to annoying and inconveniencing pilgrims went beyond the intention of state parliament.
In 2006, the parliament passed the World Youth Day Act which allowed the World Youth Day Authority to pass the annoyance clause in 2008.
In dismissing the other points of the coalition's claim, Justice French found that parts of the act banning the sale of certain items including stickers, badges and T-shirts did not infringe upon the right to free political communication.
However, the judge said the annoyance clause was invalid because it could not have been the intention of parliament to make such vague and extensive limits to free speech.
"We have interpreted the World Youth Day Act on the presumption that it was not the intention of parliament that regulations would be made under the act preventing or interfering with the exercise of the fundamental freedom of free speech," Justice French said.
Clause 7.1b of the regulation refers to "conduct that causes annoyance or inconvenience to participants in a World Youth Day event".
Justice French ruled that clause "invalid to the extent that it seeks to prevent merely annoying conduct".
WYD organisers expect protesters to be carried away with the spirit of the event and predict few serious disturbances during celebrations.
WYD coordinator Bishop Anthony Fisher said he expected there would be "very little in the way of protest" during the six-day Catholic event.
"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."
Bishop Fisher said he hoped any protests would be peaceful and there would be no need for police intervention.
"The police are very commonsensical about these things in this country," he said.
"You don't see the sort of things you see in some other situations around the world when there's demonstrations.
"If anybody does decide to engage in peaceful protest, which is something everyone has the right to do in a democratic society like Australia, (we hope) that they will do so peacefully and respectfully and that they will be respected by the police and certainly by our volunteers."