India is set to overtake China and become the world's most populous country in less than a decade - six years sooner than previously forecast, the United Nations said on Wednesday.
Fast-growing Nigeria is on course to outstrip the United States by about 2050 to become the country with the third largest population, the United Nations predicted.
The current global population of 7.3 billion is forecast to reach 9.7 billion in 2050 and 11.2 billion in 2100, slightly above the last set of U.N. projections.
Most growth will happen in developing regions, particularly Africa, according to the report World Population Prospects.
The demographic forecasts are crucial for designing and implementing the new global development goals being launched later this year to replace the Millennium Development Goals.
John Wilmoth, head of the U.N. population division, said the concentration of growth in the poorest countries would make it harder to eradicate poverty, combat hunger and expand schooling and health systems.
The world's two largest nations, China and India, have well over 1 billion people each and are likely to switch places by 2022 - six years earlier than previously forecast.
Experts predict Africa will account for more than half the world's population growth in the next 35 years.
In recent years, fertility has declined in almost all parts of the world, while life expectancy has increased significantly in the poorest countries, rising from 56 to 62 since the beginning of the century.
Declining fertility and rising life expectancy mean the world is getting greyer, and most regions will have an aging population, starting with Europe where one third of the population is projected to be over 60 by 2050, the report said.
Globally, the number of people aged 80 or over - currently 125 million - is projected to more than triple by 2050 and to increase more than seven times by 2100.
But populations in many regions are still young. In Africa, children under 15 account for two fifths of the population.
"The large number of young people (in Africa) who will reach adulthood in the coming years and have children of their own, ensures that the region will play a central role in shaping the size and distribution of the world's population over the coming decades," the report said.