NEW DELHI — Polls opened Thursday in the first phase of India's general elections, seen as a referendum on Prime Minister Narendra Modi and his Bharatiya Janata Party.

A festive spirit prevailed as men and women in colorful clothes made their way to heavily guarded voting stations in large numbers.

In the world's largest democratic exercise, voters in 18 Indian states and two Union Territories are casting ballots on Thursday, the first day of a seven-phase election staggered over six weeks in the country of 1.3 billion people.

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.

India's Prime Minister Narendra Modi addresses a campaign rally in Junagadh, India, on Wednesday. Amit Dave / Reuters

Thursday's voting is important for the BJP as it had won only 32 of 91 seats in the previous 2014 elections. It is seeking to improve its tally this time.

Modi came to power in 2014 and the party invoked its Hindu nationalist roots before the elections, with Modi at the forefront against the threat of Pakistan, India's Muslim-majority archrival. Hindus comprise about 80 percent of India's 1.3 billion people.

Even though India continues to be one of the fastest-growing economies in the world, the Modi-led government's performance on the economy has come under criticism.

The first item in the opposition Congress party's election manifesto describes a plan for creating jobs. It also promises an income subsidy program for the poorest families and for farmers.

Some 900 million people are eligible to cast ballots at around a million polling stations across India. They will decide 543 seats in India's lower house of Parliament. Voting concludes on May 19 and counting is scheduled for May 23.