Pope Benedict XVI has urged young people to reject the "spiritual desert" spreading throughout the world and to embrace Christianity to build a new age free from greed and materialism.

At a Mass before more than 200,000 young Roman Catholic pilgrims in Sydney, Benedict said on Sunday that "the world needs renewal" and challenged them to be the agents of change.

"In so many of our societies, side by side with material prosperity, a spiritual desert is spreading: an interior emptiness, an unnamed fear, a quiet sense of despair," he said.

Benedict said it was up to a new generation of Christians to build a world in "which God's gift of life is welcomed, respected and cherished - not rejected, feared as a threat and destroyed".

The aim was "a new age in which hope liberates us from the shallowness, apathy and self-absorption which deadens our souls and poisons our relationships", he said.

At Sunday's Mass, Benedict urged Roman Catholics to be agents of change [AFP]

Sunday's Mass wraps up the church's six-day World Youth Day festival in Sydney that has drawn massive crowds of pilgrims.

It has been watched on television by a global audience estimated to be in the hundreds of millions.

It was delivered at a horse racetrack filled with pilgrims who had camped out overnight.

Benedict flew over the scene early on Sunday in a helicopter to see the gathering.

He later took a slow drive through the crowd, stopping once to plant a kiss on the forehead of a toddler held up to the popemobile's window.

Pilgrims from more than 160 countries gave him a rock-star welcome, waving the flags of their nations, cheering and chanting: "Benedicto! Benedicto!" (the pope's Italian name).

Abuse apology

The Mass comes a day after Benedict made a forceful apology for the sexual abuse of children by Australia's Roman Catholic clergy, keeping up efforts begun in the US to publicly atone for what he called evil acts by priests.

In his apology, Benedict said: "I am deeply sorry for the pain and suffering the victims have endured and I assure them as their pastor that I too share in their suffering."

He said he wanted "to acknowledge the shame which we have all felt" and called for those responsible to be "brought to justice".

The acts were "evil" and a "grave betrayal of trust", he said.

But the apology was not enough to satisfy representatives of the victims of clergy sexual abuse, who said it must be backed by Vatican orders to Australian bishops to stop what they say are efforts to cover up the extent of the problem and to block survivors' attempts to win compensation.