With nearly half the world's population now living in cities, and expected to rise to 60 percent by 2020, urbanization is an unstoppable force that poses huge challenges to politicians and planners, speakers at the UN-backed conference said.

"How we plan and govern our cities will determine whether our future will be sustainable, or brutal," warned Anna Tibaijuka, UN Under Secretary-General for UN-HABITAT.

"The sustainable city is now in sight," Tibaijuka said. "Why then, are our cities becoming less and less sustainable? ... Why are urban slums, which now contain one billion people, still growing at a rate that outpaces all our attempts to deal with them?

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Tibaijuka said a lack of urban planning support for local authorities and openness in decision making represented "a failure of political will."

A UN report on the state of the world's cities officially released Monday predicted that 60 percent of the world's population will be living in cities within 14 years, sharply up from the current level.

Canadian Prime Minister Stephen Harper told the conference that the trend from rural to urban was "a powerful, irresistible phenomenon" and dismissed the "nostalgic perspective" that cities are too big, crowded and impersonal.

Instead, Harper said, healthy cities are the key to the world's future. "Throughout history, great cities have been a hallmark of successful societies."

Harper quoted late Canadian urban visionary Jane Jacobs, who argued in several of her books that societies that flourish do so on the strength of creative and workable cities.

He also stressed that, in Canada's case, cultural diversity is a strength of society and not a weakness, especially in fighting extremism and terrorism.

"The challenge for policy makers is to keep our cities healthy and strong," he told the conference meeting in this Pacific coast city.

The global forum opened to the pounding of traditional aboriginal drums, chants and the dizzying swirl of world champion hoop dancer Hal Eagleton.

More than 15,000 delegates began talks on urbanization, slums and the environment. Conference topics range from population growth and migration to the gap between rich and poor, housing, crime, terrorism and natural disasters, energy and pollution.

"We are at a turning point in history with humankind becoming urban," analyst Eduardo Moreno, chief of the Global Urban Observatory for UN-HABITAT.

"Cities are the drivers of the economy ... generate jobs, they are the places of education," Moreno told reporters.