An estimated 180,000 of the world's Catholic youths will swarm through Sydney streets on Saturday on a pilgrimage walk to a mammoth slumber party at Royal Randwick Racecourse.
The Sydney Harbour Bridge has been closed for only the third time in its history while roads in the city centre and 25 suburbs will be closed as pilgrims walk from North Sydney through the city and the eastern suburbs to the racecourse.
The walk began at 5am local time and organisers expect it to continue well into the afternoon.
Many of the pilgrims will remain at the racecourse for a sleepover ahead of the final papal mass which rounds off a week of World Youth Day celebrations.
Pope Benedict XVI will spend Saturday dedicating a new altar at St Mary's Cathedral and lunching with Australian bishops before joining the pilgrims at the racecourse at 7pm and leading them in a candlelit evening vigil.
Before his appearance, pilgrims will be entertained by an afternoon of outdoor concerts.