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Polls have opened in the first phase of India's general elections.

Voters in 18 Indian states and two Union Territories began casting ballots on Thursday, the first day of a seven-phase election staggered over six weeks in the country of 1.3 billion people.

The election, the world's largest democratic exercise, is seen as a referendum on Prime Minister Narendra Modi and his Bharatiya Janata Party. Modi supporters say the tea-seller's son from Gujarat state has improved the nation's standing. But critics say his party's Hindu nationalism has aggravated religious tensions in India.

Some 900 million people are registered to vote for 543 seats in India's lower house of Parliament.

Voting concludes on May 19 and counting is scheduled for May 23.