Voters are heading to the polls across the UK in the biggest test of political opinion since the general election.
The bitter battle to be London mayor and the fight for control of the Scottish parliament, have attracted most attention, but 16 million people are eligible to vote in council elections across England - though turnout is expected to be low.
The elections mark the first nationwide test for Jeremy Corbyn since he stormed to victory with massive grass-roots support as Labour leader last September.
But Labour had to retreat from comments Mr Corbyn made in the final days of the campaign that the party was on course for gains in the local authority elections.
Mr Corbyn changed tack, saying "predictions are not that important" as the party feared losses after Labour made nearly 800 gains under Ed Miliband when the seats were last contested in 2012.
The party's main hope of success is that Sadiq Khan takes back City Hall in London after eight years of Tory rule.
Conservative mayoral candidate Zac Goldsmith has tried to seize on the anti-Semitism controversy engulfing Labour in a bid to boost his chances after trailing in the polls.
Ukip is hoping for a breakthrough in Welsh National Assembly elections and the make-up of the Stormont power-sharing executive in Northern Ireland will also be decided.
In all, 2,700 seats will be contested in 124 English councils, and all police and crime commissioners in England will be decided.
Liverpool, Salford and Bristol also see mayoral showdowns.
Labour will also be defending Ogmore and Sheffield Brightside in two parliamentary elections.