More than half of the world's population will live in cities by 2008, most of them in developing countries, a report by the UN Population Fund says.
The UN says adequate planning is vital to prevent disaster
Urban populations are set to double in African and Asian cities over the next 30 years, warns UNFPA.
This will add 1.7 billion people to those cities, more than the populations of China and the US combined, it says.
This growth threatens the world with disaster, unless politicians undertake a radical rethink, the report says.
The report, entitled The State of World Population 2007: Unleashing the Potential of Urban Growth, says every week the number of people living in cities in Africa and Asia increases by approximately one million.
And by 2030 the UN Population Fund says the number of city inhabitants will be over five billion, or 60% of world population.
To cope, the UN says, politicians have to stop trying to prevent people moving to cities Urbanisation is inevitable and a good thing, it adds.
"What happens in the cities of Africa and Asia and other regions will shape our common future," says UNFPA Executive Director Thoraya Ahmed Obaid.
"We must abandon a mindset that resists urbanisation and act now to begin a concerted global effort to help cities unleash their potential to spur economic growth and solve social problems."
According to the report, this wave of growth in urban living is without precedent.
And while the megacities (more than 10 million people) of the world will continue to grow, the majority of the population growth will be in urban areas of 500,000 people or fewer.
Without adequate planning, the report says this growth in urban living will create huge slums, degrade the environment and radicalise the young.