Millions of Indians have started voting in a six-week election that's a referendum on Narendra Modi, the populist prime minister who has championed an assertive brand of Hindu nationalist politics and is seeking a rare third term as the country's leader.

Over 970 million voters will elect 543 members for the lower house of Parliament for five years, starting from Friday, during staggered elections that will run until June 1.

The votes will be counted on June 4.

The election is seen as one of the most consequential in India's history and will test the limits of Modi's political dominance.

If Modi wins, he'll be only the second Indian leader to retain power for a third term, after Jawaharlal Nehru, the country's first prime minister.

Most polls predict a win for Modi and his Hindu nationalist Bharatiya Janata Party, who are up against a broad opposition alliance led by the Indian National Congress and powerful regional parties.

It's not clear who will lead India if the opposition alliance, called INDIA, wins the election. Its more than 20 parties have not put forward a candidate, saying they will choose one after the results are known.