Voting is taking place for elections in England, Scotland and Wales for, in what has been dubbed "super Thursday".
Millions of voters are heading to the polls
The Scottish Parliament and local councils, the Welsh assembly and most local authorities in England outside London are up for grabs.
About 39 million people are entitled to vote, and the warm weather was expected to have boosted turnout in many areas.
Polling stations are open until 2200 BST, but many voters have cast their ballots by post in recent weeks.
In Scotland, there are elections for 129 members of the Scottish Parliament.
The election uses a form of proportional representation, with the electorate having two votes - one for a candidate in a constituency, and the second for a party. From this vote, 56 so-called "top-up" members are picked.
There are also elections in all of Scotland's local authorities.
The first results in Scotland are due to start coming in at about 0030 BST on Friday.
Wales is voting for 60 members of the assembly, again using "first past the post" and proportional representation. There are no council elections there.
The counts are expected to take place overnight.
WALES - HOW THEY STAND
Plaid Cymru: 12
Lib Dems: 6
In England 32.8 million people will be able to vote - with seats being contested in 312 local authorities. In some councils one third of seats will be up for election, in others, the whole council will be elected.
There are also mayoral elections in Bedford, Mansfield and Middlesbrough.
It is the biggest election in the local cycle in England, with about 10,500 council seats to be filled.
But more than half the counts in England have been delayed until Friday - rather than starting straight after polls close and counting into the early hours - because of a new postal voting system.
PARTIES DEFENDING SEATS IN ENGLISH LOCAL ELECTIONS
Lib Dem: 2419
Others: 1277 of which:
Mebyon Kernow: 6
Source: BBC Research
Election officers must double check at least 20% (but preferably 100%) of signatures accompanying ballot papers against those on the original applications for a postal vote.
It means a clear picture of the results will be unlikely to be available before Friday afternoon.
There are also 12 pilot areas in which electronic voting is being tried out.