By 2025 one percent of the world 's urban population will live in megacities says the UN . The most populous megacity is Brazil in Brazil , with 21 to inhabitants . Megacities in countries like India and Brazil will grow fastest over 2025 .
By 2025 10.3 percent of the world 's urban population will live in megacities says the UN . The most populous megacity is Tokyo in Japan , with 35.2 million inhabitants . Megacities in countries like India and Brazil will grow fastest over the next 15 years .