Polling booths across Britain have now opened for the third election in five years.
Polls opened at 7am and will close at 10pm
Prime Minister Boris Johnson, who gambled his premiership by triggering the vote, has sought to focus on his pledge to "get Brexit done" throughout the campaign.
Labour leader Jeremy Corbyn, his rival in the race to Number 10, has instead tried to highlight his party's credentials on the health service and other domestic issues.
Mr Johnson is expected to cast his vote in Westminster early Thursday morning while Mr Corbyn will vote in Islington.
On Monday, Mr Johnson came under fire when he pocketed a journalist's phone when asked to view a photograph of a four-year-old boy asleep on a hospital floor.
The following day, however, Labour's campaign was rocked when a member of the shadow cabinet was revealed to have criticised Mr Corbyn's election chances in a leaked recording.
A terror attack on London Bridge - which echoed a similar incident in the middle of the 2017 election - briefly disrupted the campaign, but quickly turned political as the Conservatives and Labour exchanged blows over how to deal with such threats.
The election campaign has been largely dominated by the 2016 vote to leave the European Union - with Labour pledging to give voters another say in a second referendum, while the Tories have vowed to take the UK out of the EU next month.
Liberal Democrat leader Jo Swinson said the polls showed it was still "absolutely possible" to deny the Tories an overall majority through tactical voting.
And Brexit Party leader Nigel Farage, speaking in Doncaster, said he was hoping for "very, very heavy rain" in the town on Thursday, in the belief that it would depress the votes of the other parties.
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