Voting was under way at more than 40,000 polling stations across the country. A total of 650 MPs will be elected. About 46.9 million people, including an estimated 1.5 million Indian-origin voters are registered to vote.
To form a majority in the House of Commons, one party must win 326 seats. Polls close at 10 pm UK time (2.30 am IST).
Britain last voted in a general election in 2015, when then Prime Minister David Cameron won a majority with 331 MPs for the Conservative party. Since then the referendum on Britain's membership of the European Union (EU) in June 2016 has meant a very different political landscape.
Brexit led to Cameron's resignation last year and May's selection by the Tories as their leader.
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The ongoing voting is the fourth major UK poll in three years, following the 2014 Scottish independence referendum, the 2015 general election, and the 2016 Brexit vote.
A spokesperson for the UK's National Counter Terrorism Policing headquarters has said security around polling stations is "constantly reviewed and updated by local police forces".
There were two major terror attacks during the election campaign - a suicide bombing in Manchester on May 22 that claimed 22 lives and an attack by three terrorists who drove into pedestrians and then went on a stabbing spree in London Bridge, killing eight people, before being shot dead. The Islamic State terror group has claimed responsibility for the attacks and warned of more attacks.
May had decided to overturn the UK's Fixed Term Parliament Act, which would have seen an election being held after a fixed five-year term in 2020 and called a snap general election back in April with the aim of winning a strong mandate for Brexit negotiations.
The first results in today's poll are expected to pour in before midnight local time with the final results declared by tomorrow afternoon.
The shape the new government is likely to take is expected to emerge after midnight, with a steady trickle of results until about 2 am when declarations should start to come in from across the UK.
The leader of the winning party traditionally waits for the leader of the losing party to concede defeat before claiming victory.
Many of the votes have already been cast through postal voting, which accounted for 16.4 per cent of the total electorate at the 2015 general election.
Overall turnout in 2015 was 66.4 per cent, up from 2010.
Counting will begin immediately after the polls close, with each vote counted and checked by hand.
The odds seem to be in favour of May holding on to her job as the British Prime Minister who called for snap polls 52 days ago.
It remains to be seen if the Conservative party leader's decision to call a snap general election follows the forecast patterns of the latest opinion polls and betting odds or the Jeremy Corbyn-led Labour party is able to make a dent into her slim majority in the House of Commons.
"Never before has there been a clearer choice between the parties... A choice quite simply between hope and fear," he said.
According to poll forecast, May's ruling Conservatives are believed to have an edge over the Corbyn-led Opposition Labour Party.
May's ruling Conservatives are on 44 per cent, Labour at 34 per cent, the Liberal Democrats at 9 per cent, with far- right UKIP at 5 per cent, the Scottish National Party (SNP) at 4 per cent and the Green Party at 2 per cent.
"Generally, on average, the polls tend to point toward a fairly small Tory majority, whereas the betting is more optimistic of the prime minister landing a majority of around 70 seats," said a betting expert at Ladbrokes, one of the UK's leading bookmakers.
All the major parties wrapped up their campaigning yesterday with a last-ditch effort to swing undecided voters.
May focussed on her central message of the "strong and stable leadership" that she can provide for Britain's exit negotiations from the European Union (EU).
The Liberal Democrats have been focused on their Remain- voting target seats, while the anti-EU UK Independence Party (UKIP) has stuck to its central message of ensuring there is no back-tracking on Brexit.
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