AFP explains how it works and what is at stake.
Q. Why will it take so long?
Almost 814 million adult Indians are eligible to vote, making it the biggest election in history.
Organisers say it would be impossible to operate and guard nearly 930,000 polling stations on a single day.
The local weather, festivals, harvesting period and school schedules are also taken into consideration to ensure each state goes to the polls at the most convenient date.
Q. How do people vote?
The Election Commission mandates a polling booth to be no more than two kilometres (1.
2 miles) from every voter.
India uses electronic voting machines which reduce the time taken to cast a vote and also allow for faster counting of ballots on the allotted day -- May 16.
Q. What's at stake?
India is the world's second-most populous country, a key member of the G20 global grouping and an increasingly important voice for developing countries on issues from climate change to global trade deals.