SYDNEY (AFP) - Pope Benedict XVI on Sunday urged hundreds of thousands of young Catholics to beat back a "spiritual desert" spreading through the modern world as he closed Catholic World Youth Day in Australia.

The pope celebrated an open-air mass in Sydney that organisers said drew 400,000 worshippers in the climax of a week of prayer and pop concerts during which the pontiff made a historic apology for child sex abuse by clergy.

The pope said the worshippers' youthful energy helped reinvigorate the church and urged them to become "messengers of love" to counter a world that was increasingly spiritually barren.

"The world needs this renewal," he said. "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."

The pontiff, who has repeatedly railed against consumerism and greed through the week, again warned the pilgrims to avoid "the shallowness, apathy and self-absorption which deaden our souls and poison our relationships."

Bidding "arrivederci" to the massive crowd at a Sydney racecourse, the pope announced that the next World Youth Day would be held in Madrid in 2011.

The final service in the Catholic youth festival came a day after the pontiff said he was "deeply sorry" for the "evil" of the sexual abuse of children by clergymen.

Royal Randwick Racecourse was transformed into a sea of cheering and flag-waving Catholic devotees as the pope took to a special stage with arms upraised in greeting.

Organisers and the Vatican released conflicting figures on the number of worshippers, with the Holy See putting the number at just 350,000.

But while it was clear attendance fell well short of the 500,000 predicted by organisers, Benedict said the entire event had been an unforgettable experience.

The 81-year-old pope received a bird's-eye view of the massed pilgrims from a helicopter before alighting and entering his iconic "popemobile."

He did a slow circuit of the venue, smiling and waving as mothers thrust babies up to the vehicle's large windows to bring their children closer to the papal aura.

The German-born leader of the world's 1.1 billion Catholics arrived in Australia a week ago to preside over the biggest Christian gathering on earth and was given a rock-star-style official welcome on Thursday.

The event's combination of religious services, music and barbecues drew an estimated 223,000 pilgrims, replacing Sydney's easy-going pace with an atmosphere that combined football match fever with rock concert festiveness and religious fervour.

World Youth Day was launched in 1986 by the late pope John Paul II in a bid to help stem the flow of young Catholics away from the once-dominant church in an age of growing secularism in the western world.

But this year's celebrations were partly overshadowed by a scandal over the sexual abuse of children by some Catholic clergy that has rocked the global church for years.

Amid pressure from victims, the pope on Saturday apologised for abuse in the Australian church and called for those responsible to be brought to justice.

During a mass for local clergy in Sydney's St Mary's Cathedral, he expressed his shame and made his first direct and explicit apology to victims of paedophile priests.

"Here I would like to pause to acknowledge the shame which we have all felt as a result of the sexual abuse of minors by some clergy and religious (order members) in this country," Benedict said.

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

But some activists dismissed the pontiff's apology before a group of bishops, seminarians and novices, saying words were not enough and that he should have apologised in front of sex abuse victims.

"The pope is willing to meet all sorts of disadvantaged people, but not people who have been sexually abused by the church," said John McNally, 53, an abuse victim with the support group Broken Rites.

His colleague and fellow abuse victim Chris MacIsaac said action was needed -- not rhetoric.

"He should be making sure that his bishops are putting in place adequate processes to deal with sexual abuse. The victims are not being treated fairly and justly," she told AFP.

"They should be making sure that they don't hide evidence," she said.

The pontiff will fly out of Australia on Monday.

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