The next Scottish Parliament, Welsh Senedd, London mayor and London Assembly will be determined. Sir Keir Starmer risks a major blow if the Conservatives succeed in winning the by-election for Hartlepool, a traditional Labour stronghold.
Elections have also taken place for 13 regional mayors, 39 police and crime commissioners and 143 English councils.
Counting will begin overnight in some areas with results expected from Hartlepool and certain council elections.
However due to the coronavirus pandemic counting is expected to take longer than usual, with most results coming in between Friday and Monday.
London mayor Sadiq Khan is hoping to secure re-election against Tory rival Shaun Bailey.
In Scotland the SNP is seeking the mandate for another independence referendum.
Unionist parties are aiming to weaken Nicola Sturgeon by depriving the SNP of an absolute majority.
Welsh 16 and 17-year-olds were able to vote for the first time for the Senedd election.
Labour is expected to remain the dominant party in Wales but is facing challenges from the Conservatives and Welsh nationalists.
Hartlepool has always voted Labour since the Parliamentary constituency was created in 1974.
A Conservative win here would be a huge boost for Mr Johnson and a devastating blow for Sir Keir Starmer.
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10.15pm update: Hartlepool humiliation looms for Starmer – Ex-Labour MP condemns Remainer nomination
Sir Keir Starmer will only have himself to blame if Labour loses today’s Hartlepool by-election – because the party opted to field an ardent Remainer in a constituency which voted heavily to Leave in 2016, Kate Hoey has said.
And the former Labour MP for Vauxhall – who now sits in the House of Lords as Baroness Hoey – said the “ridiculous” decision was evidence the party had left its core supporters behind.
With a poll earlier this week suggesting Labour’s Dr Paul Williams is currently trailing Tory opponent Jill Mortimer by 17 points, Sir Keir has admitted the party is scrapping for every vote.
9.55pm update: Nicola Sturgeon explodes at right wing activist outside polling station – ‘Fascist racist!’
Nicola Sturgeon has angrily confronted a right-wing heckler on election day outside a polling station in Glasgow.
Nicola Sturgeon has been confronted while leaving a polling station in Glasgow by a rival candidate for her seat, Jayda Fransen.
Ms Fransen, who is running as an independent, has links with the far-right party Britain First.
Footage shows the activist march up to the SNP leader as Ms Sturgeon gathered with party volunteers leading fiery altercations between the two.
The First Minister stood her ground, branding Ms Fransen as a “fascist” and a “racist” as the row continued down the street.
9.35pm update: Sadiq Khan’s election nightmare – Low London turnout sparks panic
Labour’s internal numbers show in-person turnout for the London Mayoral election is far lower than in 2016, raising concerns within the party’s ranks.
Figures suggest just 11.6 percent of the total turnout at the last London Mayoral election had turned up at polling stations by midday.
The figure is far lower than had been expected by the Labour Party, driving concerns of a shock result.