Thursday's polls are the UK's third in less than five years and also the first winter election to take place in December since 1923, the BBC reported.
Polling stations in 650 constituencies across England, Wales, Scotland and Northern Ireland opened at 7 a.m. (local time).
After the polls close at 10 p.m., counting will begin straight away. Most results are due to be announced in the early hours of Friday morning.
If a party wins in 326 constituencies it will have gained a majority.
A total of 650 MPs will be chosen under the first-past-the-post system used for general elections, in which the candidate who secures the most votes in each individual constituency is elected.
Anyone aged 18 or over is eligible to vote, as long as they are a British citizen or qualifying citizen of the Commonwealth or Republic of Ireland and have registered to vote.
The two main contenders, Prime Minister Boris Johnson and opposition Labour Party leader Jeremy Corbyn, made their last election pledges on Wednesday.
In interviews with the BBC, Johnson repeated his main pledges, saying: "Only if you get Brexit done (can you) move the country forward."
The Prime Minister said that he thought the election result would be "very close", and that "every vote counts".
Meanwhile, Corbyn said that there was a "greater understanding" from the public that the country "cannot go on with underfunded public services".
If there is a Labour government on Friday morning, Corbyn said the first thing he would do was "deal with the worst levels of poverty in Britain" - namely the homeless - saying: "Something must be done very quickly, very urgently and that is what we are going to do."
Other party leaders also travelled the country on the last day before the election to win support from undecided voters.
Liberal Democrat leader Jo Swinson said votes for her party could stop Brexit, adding: "Our country can be better than what Boris Johnson and Jeremy Corbyn are trying to say is the only way forward."
The Scottish National Party's Nicola Sturgeon said a vote for her party was a vote to stop further cuts to public services and to "stop Scotland being dragged out of the EU against its will".
Nigel Farage warned against Johnson's deal to leave the EU, calling for voters in Leave seats to back his Brexit Party.
Green Party co-leaders Sian Berry and Jonathan Bartley urged voters to make it a moment of political reckoning on the climate, saying their party would make sure proper action was taken to meet carbon emission targets.
Both Labour and the Conservatives have branded Thursday's vote the "most important in a generation" as the two sides have vastly different plans for Brexit and spending on public services.
( With inputs from IANS )