Pope Bendict XVI said Sunday that World Youth Day in Australia had been an unforgettable experience but warned that a "spiritual desert" was spreading in the modern world.

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The 81-year-old pontiff made the remarks as he closed the six-day Catholic youth festival before up to 500,000 worshippers at a mass at Sydney's Randwick racecourse.

"Here in Australia, this 'great south land of the Holy Spirit', all of us have had an unforgettable experience of the Spirit's presence and power in the beauty of nature," he said.

The pope urged the young pilgrims who travelled to Australia from all over the world to become "messengers of love".

"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."

He said the church needed the idealism and generosity of the pilgrims gathered in Australia so it would remain young in spirit.

'Cherish life'

He also made reference to abortion, calling on the pilgrims to cherish life.

"A new generation of Christians is being called to help build a world in which God's gift of life is welcomed, respected and cherished, not rejected, feared as a threat and destroyed," he said.

The German-born pontiff called on the faithful to usher in "a new age in which hope liberates us from the shallowness, apathy and self-absorption which deaden our souls and poison our relationships."

"Dear young friends, the Lord is asking you to be prophets of this new age, messengers of his love, drawing people to the Father and building a future of hope for humanity," he said.