India voted on Saturday in the final phase of a long-drawn general election, held in record summer heat in many parts, as both Prime Minister Narendra Modi and his challengers said they would win the polls centred mostly on inequality and religion.

The seven-phase vote, in which nearly a billion people were eligible to cast their ballots, began on April 19 and will end with polling in the last 57 seats - including in Modi's constituency in the Hindu holy city of Varanasi.

More than 100 million people are registered to vote across eight states and federal territories on Saturday, including in the northern state of Punjab and the eastern states of Bihar, West Bengal and Odisha.

"Calling upon the voters to turnout in large numbers and vote," Modi said as polls opened. "Together, let's make our democracy more vibrant and participative."

Modi is seeking a rare, third straight term as prime minister as his Hindu nationalist Bharatiya Janata Party (BJP) battles an opposition alliance of two dozen parties led by the Congress, and is widely expected to win a majority.

But he and his party have run into a spirited campaign by the opposition alliance called 'INDIA' or the Indian National Developmental Inclusive Alliance, sowing some doubt about whether Modi's expected victory would come easily.

Scorching summer temperatures with unusually high heatwaves have compounded voter fatigue in the majority-Hindu country of 1.4 billion people, where unemployment and inflation are the main concerns for voters.

Nearly two dozen election officials died of suspected heatstroke in Bihar and Uttar Pradesh ahead of the vote on Friday, authorities said.

Modi and Congress leader Rahul Gandhi have both predicted heavy defeat for each other and said their respective alliances were set to form the next government.

TV exit polls expected after the conclusion of the vote will project how well or poorly the parties are faring ahead of the results, due on June 4. Exit polls, however, have a patchy record in India and have been widely off the mark previously.

Modi began his re-election campaign by focusing on his achievements over the last 10 years but soon switched to mostly targeting the opposition by accusing them of favouring India's minority Muslims, comprising roughly 200 million of the population.

This change of tack, analysts said, was likely aimed at firing up his Hindu nationalist base after a low turnout in the first phase sparked concerns that BJP supporters were not voting in large numbers.

The opposition campaign has largely focused on affirmative action and saving the constitution from what they say is Modi's dictatorial rule.