These emerging megaregions pose challenges to governments, according to UN-Habitat's bi-annual State of World Cities report.
"The largest of these, says the report - launched today at the World Urban Forum in Rio de Janeiro - is the Hong Kong-Shenhzen-Guangzhou region in China, home to about 120 million people. Other mega-regions have formed in Japan and Brazil and are developing in India, west Africa and elsewhere.
The trend helped the world pass a tipping point in the last year, with more than half the world's people now living in cities.
The UN said that urbanisation is now 'unstoppable'."