For centuries Delhi has served as the capital for kingdoms and empires. But India’s first city will soon get a fresh label: the world’s most populous urban centre.

Delhi is already home to 29 million people, according to United Nations estimates, and is forecast to add an eye-watering 870,000 people per year during the next decade as a growing share of the Indian population settles in cities. By 2028, Delhi’s population will overtake that of Tokyo, currently the world’s biggest urban agglomeration with 37 million people.

Smog envelopes Delhi last week, when air pollution levels were at near-record levels. Credit:Bloomberg

India's landlocked megacity, which only reached the 20 million mark a decade ago, is "projected to become the most populous city in the world with 39 million people in 2030”, says the latest World Urbanisation Prospects report by the UN’s Department of Economic and Social Affairs.

A huge amount has been invested in India’s vast capital in recent decades. In a little more than 20 years Delhi has constructed an impressive metro rail network which now compares in length to the London Underground. Nilesh Rajadhyaksha is an urban specialist at India’s National Institute of Urban Affairs working on a 20-year masterplan for Delhi. In his office in the city, he tells the Herald and The Age that it has emerged as “one of the most complex urban systems anywhere in the world”.