Cities with population of over 10 million people are categorized as megacities.

The United Nations' Department of Economic and Social Affairs said in a March 2010 report that by 2025, there will be 29 megacities globally from 21 two years ago, with the majority in developing countries.

As the world’s population reaches seven billion on Oct. 31, 2011, here’s a list of world’s top megacities. Start the slideshow to view the famous skylines of the world.