Mumbai is set to replace Tokyo as the world's most populous city by 2020, according to a study released by the Washington-based Population Institute. Mumbai, currently home to about 18 million people, will over the next two decades see its population grow to some 28.5 million, the institute said in its annual overview of world population trends on Friday. Tokyo is expected to be pushed into second place, with a population of 27.3 million in 2020.