After six weeks of campaigning and debate, people have been casting their vote at around 50,000 polling stations.
Polls close at 22:00 BST with results from the first constituencies expected before midnight and the final result due on Friday afternoon.
As well as the general election, there are more than 9,000 council seats being contested across 279 English local authorities.
Mayors will also be elected in Bedford, Copeland, Leicester, Mansfield, Middlesbrough and Torbay.
UKIP leader Nigel Farage, Labour leader Ed Miliband, Greens leader Natalie Bennett, SNP leader Nicola Sturgeon, Conservative leader David Cameron, Liberal Democrat leader Nick Clegg and Plaid Cymru’s Leanne Wood have all cast their votes.
In Northern Ireland DUP leader Peter Robinson and Sinn Fein’s Martin McGuinness in Northern Ireland have voted, as have SDLP leader Alasdair McDonnell, Alliance Party leader David Ford and Ulster Unionist leader Mike Nesbitt.
A total of 650 Westminster MPs will be elected, with about 50 million people registered to vote.
The weather has been dry and mild across much of the UK, with isolated showers in some parts of the north of England.
The local votes taking place mean that nearly every voter in England – excluding London where there are no local elections – will have been given at least two ballot papers when they enter polling stations.
Some votes had been cast before Thursday through postal voting, which accounted for 15% of the total electorate at the 2010 general election, when the overall turnout was 65%.
For the first time, people have been able to register to vote online.
Most polling stations are in schools, community centres and parish halls, but pubs, a launderette and a school bus are also being used.
Polls close at 22:00 BST, but officials say anyone in a polling station queue at this time should be able to cast their vote.