Speaking during a mass at St Mary's cathedral, the pope said: "I am deeply sorry for the pain and suffering the victims have endured.
"These misdeeds, which constitute so grave a betrayal of trust, deserve unequivocal condemnation.
"Those responsible for these evils must be brought to justice."
There was no immediate confirmation of whether the pope would meet the victims of clergy abuse in person.
Support groups have criticised the speech. Campaigners estimate the number of victims to be in the thousands.
Broken Rites Australia, a group that supports victims of church sex abuse, said in a statement: "Sorry is not enough. Victims want action, not just words."
Broken Rites said the pope should apologise in person to individuals.
The pope will lead a mass on Sunday before an estimated crowd of more than 200,000 that will mark the culmination of the World Youth Day festival.
He has expressed regret before about the clergy abuse scandal that has rocked the church in recent years, notably during a visit to the US in April when he also met privately a small number of victims.