The vote could have far-reaching implications for the fate of the opposition Labour Party and for Scotland's future in the UK.
Vote counting was underway in the UK after polling stations closed on Thursday evening.
British voters headed to the polls on Thursday to elect new mayors in London and Manchester, new members to the Scottish and Welsh parliaments and hundreds of local councillors.
It was the biggest set of votes since the 2019 general election with polls taking place for the Scottish Parliament, the Welsh Senedd and 143 councils and 13 mayors in England. It is being dubbed "Super Thursday."
The vote could have far-reaching implications for the future of the opposition Labour Party and for Scotland's status in the UK.
In the north-eastern constituency of Hartlepool, a by-election is taking place following the resignation of the previous MP Mike Hill.
The seat has been held by Labour MPs for almost 60 years but polls suggested it could be lost to the ruling Conservative party in this election; a move which would deliver a harsh blow to the leader of the Labour Party, Keir Starmer.
In Scotland, the Scottish National Party is likely to push for another referendum on independence if they secure a majority.
Former First Minister of the SNP Alex Salmond launched his own pro-independence party following a dispute with the current leader, Nicola Sturgeon.
UK prime minister Boris Johnson has called the bid for independence "reckless" and "irresponsible". Voters will elect 129 Members of the Scottish Parliament.
Scottish voters rejected independence in 2014, with the 'No' side winning with 55.3 percent of the vote.
London Mayor Sadiq Khan is also seeking re-election against a backdrop of economic decline in the capital city due to the pandemic and Brexit.
About 5,000 seats are being contested in 143 English local council elections. The make-up of the 60-seat Senedd in Wales is also being voted on. In other ballots, voters will choose 39 police and crime commissioners, 25 members of the London Assembly and 13 directly elected mayors.
Results are expected to arrive over several days, with coronavirus restrictions slowing the counting process.