This is the second general election Britain has witnessed since its Brexit referendum in 2016.
Elections in the UK traditionally take place every four or five years, but members of parliament voted in October for a second snap poll in roughly three years.
It is also the first winter election since 1974 and the first to take place in December since 1923.
Thousands of polling stations, taking up space in community buildings, schools, libraries, shops, public houses and mobile huts, will operate until 22 p.m. Thursday. After they close, counting will begin immediately and most results will come out Friday.
The 46 million registered voters in Britain will be handed slips of paper containing the names of candidates in their respective constituencies, and some of them have already voted via postal votes, waiting to be added to the final tally.
The general election was called in a bid to break the Brexit deadlock, which has gripped Britain ever since the seismic 2016 referendum vote to leave the European Union. After struggling to lead a minority administration, Conservative Party leader Boris Johnson hopes to secure a majority government that will enable him to take Britain out of the bloc on Jan. 31.
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