India began voting on Friday in the world's largest election as Prime Minister Narendra Modi seeks a historic third term in office on the back of growth, welfare, his personal popularity and Hindu nationalism.

The vote pits Modi's Bharatiya Janata Party (BJP) against an alliance of two dozen opposition parties which is challenging him with promises of increased affirmative action, more handouts and what they say is the need to save democratic institutions from Modi's dictatorial rule.

The gigantic exercise involving almost one billion voters will be spread over seven phases across the world's most populous country at the peak of summer. It ends on 1 June and votes will be counted on 4 June.

On Friday, in the largest of the seven phases, 166 million voters in 102 constituencies across 21 states and territories will vote, including in Tamil Nadu in the south, Arunachal Pradesh on the Himalayan frontier with China, and the most populous Uttar Pradesh in the north.