The report predicted there will be 27 "megacities" with at least 10 million residents by mid-century compared to 19 giant metropolitan areas today. But it said at least half the urban growth in coming decades will be in smaller cities that now have less than 500,000 people.
According to the latest U.N. estimate last year, the world's population is expected to rise from 6.7 billion in 2007 to 9.2 billion in 2050. Over the same span, the new report said, the total population of urban areas is projected to rise from 3.3 billion to 6.4 billion.
"The urban areas of the world are expected to absorb all the population growth expected over the next four decades while at the same time drawing in some of the rural population," the report said. "As a result, the world rural population is projected to start decreasing in about a decade, and 600 million fewer rural inhabitants are expected in 2050 than today."
Hania Zlotnik, head of the U.N. Population Division, expressed hope that increasing urbanization "will go hand in hand with economic growth."
More than 70 percent of the populations in Europe, North America and richer developed nations in other regions already live in urban areas, while only 39 percent of Africans and 41 percent of Asians were in urban areas last year.
"During 2008, for the first time in history, the proportion of the population living in urban areas will reach 50 percent," the report said, adding that "the level of urbanization is expected to rise from 50 percent in 2008 to 70 percent in 2050."
By mid-century, Asia is projected to see its urban population increase by 1.8 billion, Africa by 900 million and Latin America and the Caribbean by 200 million, it said.
Zlotnik said the U.N. expects Africa to reach 50 percent urbanization between 2045 and 2050. Asia, if it continues to urbanize as rapidly as it's doing now, especially because of the rapid urbanization in China, is expected to become 50 percent urbanized around 2020-2025," she said.
China, the world's most populous nation, is 40 percent urban now, Zlotnik said. The U.N. expects its urban population to reach more than 70 percent by 2050, she said.
By contrast, the world's second most populous nation, India, has just over 300 million urban residents, or 29 percent of its population, Zlotnik said. By 2050, it is expected to have 55 percent of the population, about 900 million, in cities.
She added that disparities in urbanization will remain. Burundi and Papua New Guinea, for example, have only 10 percent of their people living in urban areas, while the small city state of Singapore is 100 percent urban, Zlotnik said.