Labour's Sadiq Khan appears on course to break the Conservatives' eight-year hold on the London mayoralty, with a live count of votes indicating a clear lead.
The Tooting MP has been the frontrunner in the counting process ahead of Tory challenger Zac Goldsmith, according to first preference votes collated using an electronic system.
An official result is not expected until after midnight at City Hall, once first and second choice votes have been counted.
But the early signs suggest Mr Goldsmith's campaign, labelled "vile" by Labour leader Jeremy Corbyn, has struggled to resonate across the capital. Richmond Park MP Mr Goldsmith and Prime Minister David Cameron sought to link Mr Khan with Muslim "extremists" in the bitter race to succeed Boris Johnson.
A source close to Mr Khan's campaign said they are "optimistic" about the result although they adopted a cautious tone with votes still to be counted.
The Green Party's Sian Berry is also seeking to secure third place in the contest ahead of Caroline Pidgeon of the Liberal Democrats and Ukip's Peter Whittle.