Voting ended on Sunday in India's most acrimonious election in decades that will decide whether Hindu nationalist Prime Minister Narendra Modi gets a second term in power.

As the final polling booths closed, a huge security cordon was thrown around the voting machines and boxes of paper votes used in the 542 seats for the world's biggest election before the official count starts on Thursday.

Tens of thousands of police and paramilitaries on duty in West Bengal state were a symbol of the mounting tensions between Modi's right wing Bharatiya Janata Party and the opposition during the six weeks of voting.

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Long queues formed outside polling stations in the eastern state but the BJP and its rivals again accused each other of using violence, fraud and intimidation.

An improvised bomb was thrown at one Kolkata polling station and security forces intervened to stop BJP, communist and other groups blocking different booths across the state capital that was hit by two days of street battles last week.

Modi's constituency in Varanasi, the holy city in Uttar Pradesh state, was also among those to vote.

Conjoined twins Sabah and Farah twins voted in the Bihar capital of Patna and 102 year old Shyam Saran Negi, who has taken part in every Indian vote since independence in 1947, cast his ballot in mountainous Himachal Pradesh state, highlighting the huge diversity of the exercise.

But most attention has focused on the BJP campaign to project Modi's strongman image, playing up recent cross-border air strikes against Pakistan.

The opposition, led by the Congress party and its leader Rahul Gandhi, have accused him of pursuing divisive policies, neglecting the economy and leaving many farmers in ruin.

Modi and Gandhi have hurled insults at each other on a near daily basis with the prime minister calling his rival a “fool” while Gandhi derides Modi as a “thief”.

Jaded voters

The animosity has taken a toll on voters.

“All the abuse and misconduct claims suggest that standards in Indian politics have slipped badly,” Asit Banerjee, a history teacher in Kolkata, said as he queued to vote.

“Endless mudslinging and bitter comments pervaded the campaign. We are losing hope in a democracy, it is time for a reset,” the 60-year-old told AFP.

Writing in the Hindustan Times, political commentator Karan Thapar said Modi's message “played on our insecurities and strummed upon our deep inner fears”. He also criticised Gandhi's campaign.

Pollsters say Modi remains personally popular but his party's overall majority is at risk from a backlash against the government.

The 68-year-old Modi has held 142 rallies across India during the campaign, sometimes five a day, but pollsters say the BJP could lose dozens of the 282 seats it won in its 2014 landslide.

$7 billion vote

On Saturday Modi, dressed in a long robe and saffron sash, trekked to a Himalayan shrine to meditate, with images showing him seated on a bed inside a holy cave in the country's north.

Hoping to become the fourth member of Nehru-Gandhi dynasty to lead India, Rahul Gandhi has struggled to make himself heard above the din of the BJP's campaign juggernaut.

The Delhi-based Centre for Media Studies estimates that the outlay on this election could top $7 billion, making it one of the priciest contests globally — with the lion's share of the spending by the BJP trying to woo India's 900 million eligible voters.

Lots of it has been spent on social media, with the parties using armies of “cyber warriors” to bombard India's hundreds of millions of Facebook and WhatsApp users with messages.

Fake news and doctored images have abounded, including of Gandhi and Modi having lunch with Imran Khan, prime minister of arch rival Pakistan, or of a drunk Priyanka Gandhi, a politician and the sister of Rahul.

Violence has also broken out. Maoist rebels killed 15 troops and their driver in the western state of Maharashtra on May 1, the latest attack in a decades-long insurgency.

Gandhi, 48, has tried several lines of attack against Modi, in particular over alleged corruption in a French defence deal and over the plight of farmers and on the economy.

Modi's government has fallen short on creating jobs for the million Indians entering the labour market every month, the shock introduction of a cash ban in 2016, while Indian banks are struggling with huge bad debts.