According to a UN forecast, India will overtake China as the world's most populous country this Friday, with more than 1.4 billion inhabitants. Large Indian cities like Mumbai are already saturated. More than 20 million people live in the country's economic capital, in an area barely six times larger than central Paris. Mumbai is the victim of excessive urbanisation, with rents soaring and wealth gaps widening between the upper classes and poorer residents, especially those living in slums. Our correspondents report.