Voters across the UK are going to the polls in a series of national and local elections as well as a referendum to decide the way MPs are elected.
Elections for the Scottish Parliament, the Welsh Assembly and the Northern Ireland Assembly are being held, as are polls in 279 English councils.
The public are also being asked if they want to keep the first-past-the-post system for Westminster elections or switch to the alternative vote.
Polls are open from 0700 to 2200 BST.
In addition, local authority elections are being held in Northern Ireland while there is a UK parliamentary by-election in the constituency of Leicester South.
Contests are also taking place to elect mayors in Leicester, Mansfield, Middlesbrough, Torbay and Bedford.
David Cameron arrived on foot at the Methodist Central Hall in Westminster to vote in the AV referendum.
Labour leader Ed Miliband accompanied his fiancee, Justine Thornton, to a polling station in north London.
Mr Miliband has already voted by post in the referendum and in council elections in Doncaster, where he is an MP.
Deputy Prime Minister and Lib Dem leader, Nick Clegg, cast his vote in Sheffield.
More than 90 people cast invalid votes in Stockport after the wrong ballot papers were delivered to polling stations.
The borough council said it was trying to reach those affected to ask them to vote again.
Meanwhile, a polling station in Llandeilo, Carmarthenshire was closed for two hours after two escaped dogs bit four voters.
The BBC weather forecast suggests that most of the UK will remain dry on Thursday with above average temperatures, but that there is a chance of rain in Northern Ireland.