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.

New York
The skyline of New York is seen October 15, 2011. New York is a classed as a megacity, those with a population of over 10 million people. 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. The world's population will reach seven billion on 31 October 2011. REUTERS/Gary Hershorn
Moscow, Russia
Moscow's skyline shows the city's Kremlin, Ministry of Foreign affairs and Moscow City business district on October 18 2011. Moscos is a classed as a megacity, those with a population of over 10 million people. REUTERS
Shanghai, China
Vehicles drive on flyovers during the evening rush hour in central Shanghai December 29, 2010. REUTERS/Aly Song
Sao Paulo, Brazil
Brazil will replace Great Britain as the world’s sixth-largest economy by the end of the year, according to a forecast by the Economist Intelligence Unit REUTERS/Paulo Whitaker
Istanbul, Turkey
The skyline of Istanbul shows the illuminated Bosphorus Bridge October 21, 2011. Istanbul is a classed as a megacity, those with a population of over 10 million people. REUTERS/Murad Sezer
Buenos Aires, Argentina
Overview of Buenos Aires' 9 de Julio Avenue Reuters/Enrique Marcarian
Rio de Janeiro, Brazil
Rio de Janeiro, Brazil. REUTERS/Bruno Domingos
Cairo, Egypt
A view of the city skyline and River Nile from the Cairo Tower building, October 27, 2011. The world's population will reach seven billion on 31 October 2011, according to projections by the United Nations. REUTERS/Amr Dalsh
Jakarta, Indonesia
An aerial view of Indonesia's capital city of Jakarta, October 21, 2011. Jakarta is a classed as a megacity, those with a population of over 10 million people. REUTERS/Supri Supri
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