Pope Benedict XVI on Thursday warned Catholics of the perils of pop culture and pillaging the earth's resources after a rapturous welcome at the world's biggest Christian festival in Australia.

Speaking against the spectacular backdrop of Sydney's famous harbour, the pontiff told hundreds of thousands of pilgrims in Australia's biggest and trendiest city that "something is amiss" in modern society.

"Our world has grown weary of greed, exploitation and division, of the tedium of false idols and piecemeal responses, and the pain of false promises," the pope said after a welcoming ceremony by Aborigines in tribal paint.

Benedict told a vast sea of youths from around the world, gathered under a forest of national flags for World Youth Day, that humanity was squandering the earth's resources to satisfy its insatiable appetite for material goods. In one of his strongest-ever messages on the environment , the pope spoke poetically of his 20-hour flight from Rome to Australia, saying the wondrous views from his plane evoked a profound sense of awe.

But the 81-year-old pontiff told his young audience that the planet's problems were also easier to perceive from the sky. "Perhaps reluctantly, we come to acknowledge that there are scars which mark the surface of our earth - erosion, deforestation, the squandering of the world's mineral and ocean resources in order to fuel an insatiable consumption," he said.

Earlier, shouts of "Viva, Papa" rang out over the harbour as a "boat-a-cade" of 13 vessels led by a water-spouting fire tug and flanked by bodyguards on jet skis glided past Sydney's iconic Opera House and Harbour Bridge en route to the pope's World Youth Day debut.