Voters in the UK are electing a Scottish Parliament and legislatures in Wales and Northern Ireland, as well as choosing many English local authorities, including a new London mayor to replace Conservative Boris Johnson.
The London mayoral race sees Labour Party candidate Sadiq Khan up against Conservative Zac Goldsmith. Pollsters have made Khan the favorite to become the city's first Muslim mayor.
Polls also put Khan, 45, and Goldsmith, 41, well clear of the 10 other candidates in the field.
"Hopefully, if I win, I'll be the mayor that unites our city again, that brings communities together," Khan said on the final day of campaigning Wednesday.
This follows a bad-tempered campaign that saw Goldsmith accuse his rival of sharing platforms with Islamic extremists.
Khan, the son of a bus driver from Pakistan, in return has accused the wealthy environmentalist Goldsmith of divisive campaigning in one of the world's most multicultural cities. More than 1 million Muslims live in the city of 8.6 million.
Labour on the ropes
A victory for Khan would be a bright spot in what could be a poor day for Labour, out of office nationally since 2010. Labour, under left-wing leader Jeremy Corbyn, is divided and allegations of anti-Semitism raised last week haven't helped its cause.
During a heated parliamentary debate on Wednesday, Prime Minister David Cameron accused Khan of sharing "a platform with an extremist who called for Jews to drown in the ocean."
Corbyn in turn accused the Conservatives of "smearing" Khan, adding that one of the men Cameron had accused Khan of sharing a platform with had also been close to Goldsmith.
In Corbyn's first electoral test since taking over the party last September, analysts say Labour could lose dozens of seats in some of its traditional strongholds. Corbyn predicted last week that Labour would not lose seats.
Mayors of London, which is home to the City financial district, are responsible for areas such as policing, transport, housing and the environment.
Scotland, Wales and Northern Ireland
In Scotland, the pro-independence Scottish National Party (SNP) is set to win a majority of seats in the Edinburgh parliament and retain governmental power, with Labour at risk of sinking to third place behind the Conservatives.
Labour dominated Scottish politics for much of the 20th century, and a third place would be another step in what appears to be its gradual evisceration in the country.
In Northern Ireland, which has its own set of political parties reflecting the Catholic-Protestant divide, rivals are competing to see whether the Catholic side can overtake the territory's dwindling Protestant majority for the first time.
The party with the most assembly seats always receives the top post. The incumbent is Democratic Unionist leader Arlene Foster (pictured), Northern Ireland's first female leader.
Polls suggest that the Irish nationalists of Sinn Fein will narrow the gap on the Democratic Unionists, but fall short of overtaking the Protestant side of the house.
Polls are open until 2100 GMT and the results are due Friday.
jbh/sms (AP, AFP, Reuters)