World Youth Day celebrations will culminate today with organisers expecting up to 500,000 people at the final mass, to be led by Pope Benedict XVI.

The faithful began streaming into Sydney's Randwick Racecourse and Centennial Park precinct yesterday, many after a nine-kilometre pilgrimage through the city, all eager for a prime position for the mass, which starts at 10am (AEST).

The massive main stage is flanked by giant TV screens, reminiscent more of a rock concert than a religious service.

The pontiff will fly over the site in a helicopter before touring through the crowds of people in the Popemobile on his way to the stage.

He will deliver the homily and a separate address during the service, which will also see the confirmation of 24 young people.

Around 180,000 of those attending the final service made the nine-kilometre pilgrimage to the site from North Sydney yesterday.

They took part in a late night vigil before braving the cold to sleep out under the stars.

Addressing the vigil, the Pope said he was "filled with joy" to be with the pilgrims.

Some slept on air mattresses, others just on a ground sheet with their sleeping bag, but all depended on the fires of faith to keep them warm on a chilly winter night.

South African pilgrim Shane Govender, 16, said it was a great opportunity to worship together.

"It's touching, so many people of the same age praying together," he said.

"It makes me feel so proud of us when so many other people our age are into drugs, smoking or drinking."

The Pope will return to St Mary's Cathedral after the mass, attending a meeting with the benefactors and organisers of World Youth Day later this evening.

AAP