Thousands of experts, politicians, slum-dwellers and activists are meeting in Vancouver hoping to plan a way forward for the world's growing cities.
Vancouver's mix of skyscrapers, sea and forests is widely admired
The World Urban Forum will discuss ways to make city life sustainable against a backdrop of rising urban populations.
United Nations estimates suggest more than half of the world's people will live in urban areas by the end of 2007.
With about a third of city dwellers living in slums, urban poverty is high on the agenda at the five-day forum.
The meeting, backed by the UN settlements agency UN Habitat, opened with a lavish ceremony featuring drumming and chanting from Canadian aboriginal people.
A crowd of thousands watched and listened to a warning that many poor people flock to cities in the hope of a better life but find conditions worse than those they left behind.
"Many of the migrating millions realise their dreams and build better lives for themselves, their families and their communities," said Canadian Prime Minister Stephen Harper.
"For others, the road to the city leads to poverty, homelessness and tragedy."
Weight of numbers
UN estimates suggest that two-thirds of the world's population will live in cities within 50 years.
UN Habitat says increased urbanisation has created a range of serious issues, including access to clean water, sanitation, shelter, urban poverty, HIV/Aids and problems with urban governance.
Ahead of the forum UN Habitat also released a report detailing the numbers of city-dwellers who live in slum districts - now thought to make up one-third of the world's urban population.
Already, rapid growth means an estimated 180,000 people are being added to the world's urban population each day.
There are also fears of new epidemics, such as gang violence or the impact of natural disasters on shoddy housing.
Many cities have grown exponentially, with a total of 20 cities now boasting populations of over 20 million, compared to just two in 1950.
The numbers living in slums have also spiked, being worst in sub-Saharan Africa, where slums make up more than 70% of the urban population.
The 2006 forum, the third meeting of its kind, is being held in Vancouver, the western Canadian city widely lauded for its high quality of life.
It comes 30 years after the first UN-backed meeting to discuss human settlement was also held in the city.
An estimated one third of city dwellers live in slums
During the forum, delegates from around the world will take part in discussions and networking sessions designed to promote positive dialogue and co-operation between key players in urban development and city management.
"Urbanisation around the world is occurring at an ever more rapid pace, and we face important challenges to ensure that this happens in a sustainable way," said Diane Finlay, Canadian minister for human resources and social development.
But local officials doubt that even Vancouver - home to 600,000 people, and 2.2 million around the city - can maintain its high standards without making radical changes.
They have proposed measures to transform the city by reducing the "eco-footprint" of the city's inhabitants.
"If everybody lived in the world the way [Vancouver does], we would need three to five planets to support us all," Greg Searle of environmental organisation One Planet told the AFP news agency.