According to the report, by the year 2050 there will be 6.4 billion people living in cities, up from 3.3 billion now. The world's total population is expected to rise to 9.2 billion in 2050 from the current 6.7 billion.

The world's most developed regions -- Europe, North America and Oceania -- have far more people in cities than in the countryside, as do Latin America and the Caribbean. Africa and Asia are exceptions but have most of the world's people.

As urbanization increases, the world's total rural population is expected to begin declining in around a decade and should fall to 2.8 billion people in 2050 from 3.4 billion in 2007, the report said.

Some countries, like India -- home to two of the world's biggest metropolises, Mumbai and Delhi with 19 million and 18.8 million people respectively in 2007 -- aim to slow down urbanization by encouraging development of rural areas.

Despite the challenges urbanization poses for governments, Hania Zlotnik, head of the U.N. Population Division, told reporters urbanization is often a sign of a lively economy.

"Governments would be well advised that urban growth is a proof of economic dynamism," Zlotnik told reporters.

Still, intense urbanization and the expected addition of eight new "megacities" -- cities with 10 million or more inhabitants -- by the year 2025 will pose new challenges.

Governments need to make sure large urban populations have access to basic services, above all health care, Zlotnik said.

Asia and Africa are still mostly rural but will see booming urban populations over the next few decades. Both have around 40 percent in cities and 60 percent in the countryside now.

But this is steadily changing. Half of Africa's population will be in cities by between 2045 and 2050 while Asia will reach that point between 2020 and 2025, Zlotnik said.

MEGACITIES

Around 40 percent of China's population is in cities now, a figure that is expected to exceed 70 percent by 2050, when over 1 billion people will be living in Chinese cities, she said.

By 2025, China's booming foreign investment center Shenzhen, which borders Hong Kong, will join Beijing and Shanghai as China's third megacity with 10.2 million people, the U.N. report projected.

The world's second-most-populous country, India, has only 29 percent of its population in cities at the moment. By 2050, India will have 55 percent of its people in urban centers.

"India is expected to urbanize much less than China and therefore it's expected to remain the country with the world's largest rural population," Zlotnik said.

But India will get two new megacities to join Mumbai and Delhi by 2025 -- Calcutta, which will have an estimated 20.6 million people, and Madras with 10.1 million.

By 2025 there will be 27 megacities and Europe will add only one more to the list -- Paris. It will have an estimated 10 million people, making it number 27 on the list. Of the 19 megacities today, the only European metropolises are Moscow and Istanbul.

Tokyo is projected to remain the most populous city in the world. With 35.7 million people in its urban agglomeration at last count, this should rise to 36.4 million by 2025, it said.

Africa currently has only one megacity -- Egypt's capital Cairo. Joining the ranks of megacities by 2025 will be Kinshasa in the Democratic Republic of Congo and Lagos, Nigeria.