Conservative Boris Johnson was leading the race to run London as counting continued on Friday evening.
Labour rival Ken Livingstone is trailing behind on first preferences after a bitter campaign that has seen the two men clash furiously in public and in private.
Liberal Democrat Brian Paddick faces the embarrassment of potentially being pushed behind into fourth or fifth place as the Green's Jenny Jones currently stands in third while Independent Siobhan Benita is close behind him.
Although election organiser London Elects is not releasing precise figures for ballot papers counted, graphs on its website show a clear advantage for Mr Johnson in the first round of the mayoral contest. If no candidate takes more than 50% of the vote it will go to a second round.
The Conservative candidate's standing, however, is not being replicated by his party in London.
Labour is ahead in the vote for London-wide Assembly candidates followed by the Tories while the Lib Dems are in fourth place behind the Greens.
London Elects' graphs put Labour ahead in eight of the Assembly's 14 first-past-the-post constituencies, with Tories on course to take the other six.
This would mean a two-seat loss for Conservatives and the Assembly looked almost certain to remain under no overall control.
In the five seats declared so far, four were Conservatives and one Labour.
Labour leader Ed Miliband refused to concede Labour's defeat in the mayoral race.
He said: "It has been a closely fought campaign in London. We will see what the results are."
Shadow chancellor Ed Balls acknowledged that Mr Johnson might win "despite his association with David Cameron and George Osborne, rather than because of it".
But with the party doing well in the London Assembly contests, he told BBC: "It certainly suggests that Labour, when it's Labour, it's doing very well compared to the Conservatives across London and we've seen that in by-elections as well.
"But there was a personality contest in which Boris Johnson said 'Vote for me even though I'm a Tory' and it looks like he may... we'll wait and see, but if he wins I think it will be despite his association with David Cameron and George Osborne, rather than because of it.
"I backed Ken Livingstone, I think he was the best mayor London's had so far. I think he would be a better mayor for London on Tube fares and jobs and housing than Boris Johnson who, let's be honest, is a bit of a joker, a bit of a buffoon."
As more votes were counted Labour claimed predictions that Mr Johnson would storm to victory were looking more uncertain.
Shadow olympics minister Tessa Jowell, chair of Mr Livingstone's campaign, told BBC News there had been an assumption in the media, based on polling, that the Tory candidate would "walk away with it".
"It's quite different now," she told the BBC. "It's obviously a very, very tight race, tighter than I thought it was going to be, and let's see."
Communities minister Bob Neill, who was in the press room at City Hall, said: "A Boris victory would be good news for London. There would be continued value for money in the way that the city's run, continuation of imaginative policies, continued investment in London's transport network, and, in particular, improvements in the jobs agenda."
Mrs Jowell added: "It's not just the last few hours, but the last few months for the Tories have been absolutely catastrophic."
So over 41 hours after polls opened on Thursday morning, Boris Johnson is mayor of London and we can all go to bed to dream analytical dreams about what the results mean for the country.
Probable ETA of a declaration is about an hour, hopefully less (please)
London Elects statement:
We have results in from 13 of the 14 constituencies in the Mayor of London and London Assembly election. The final constituency is Brent & Harrow. The Greater London Returning Officer has been in touch with the CRO for Brent & Harrow to establish the reason for a delay. All batches of ballot papers were registered and scanned. Two batches went to storage without some ballot papers being manually entered as required. Manual entry is required when a scanner cannot read a ballot paper – for example if a ballot paper is damaged. It is not an issue with the scanners. The issue was identified during the verification stage. These two batches are being re-processed. To make this happen as quickly as possible we have separated out into several smaller batches. This is why the progress screens appear to show a changing number of verified ballot papers.
We will declare as soon as possible but it is obviously important that every vote is counted.
Well we don't entirely know. The latest is that at least two ballot boxes were found unscanned at the Brent count, and they're now being counted by hand, for some reason.
There are rumours that Ken's team has asked for a full recount in Brent, which would delay things for another few hours.
London elects thinks it'll be only another 20 mins, but they've been saying that for nearly two hours.
|@ tnewtondunn : The Sun's pollster Peter Kellner of YouGov calls it - "Boris has won. He has too big a lead for Ken to overturn".|
It's leading to a sharp narrowing in the polls between the two candidates. On 1st preference votes Boris now has 44% and Ken 40% - 8 out of 14 constituencies have now declared.
Here's the latest.
People are talking about 50/50 odds between Ken and Boris. The atmosphere at City Hall has changed dramatically in the past half hour.
Ed Miliband, speaking in Southampton, has declared that Labour is "coming back".
This comes after Labour took control of Southampton from the Conservatives.
He railed against the "out of touch government in Westminster".
"David Cameron promised change, not for the better but for the worse. Today in Southampton, it is a rejection of the economic failure of this government, and the unfairness of this government". he added
He also boasted that Labour is "beating back" the SNP in Scotland.
He recognised the low turnouts, saying it showed there was "still more to do".
Conservative MP Peter Bone has said that the PM has 'one hand tied behind his back'.
"Worst of all, we've got the Liberals holding us back" he added
The results signified the "beginning of the end of the coalition", he told the BBC.
"We're seeing Boris winning as a Conservative in London, yet the Coalition losing against Labour. So the message must be let's come out of this coalition as soon as possible." he said
Liam Byrne, who quit the Shadow Cabinet in preparation for running for Birmingham Mayor, has just tweeted this
|@ LiamByrneMP : A huge thanks to ALL who supported Yes2Mayor, Albert and me: our job now is to get 4-square behind our brilliant new Lbr council #brummayor|
|@ SmithTonyD : Brum result: Leader 120,611, Mayor 88,085. 57.8 Per cent "no" #BrumVote12|
|@ oflynnexpress : Brum votes No to a Mayor. Has anyone noticed people like voting No in refs? No2EU|
With 56 of 79 seats declared, Labour currently lead over the SNP by 29 to 20.
Barring a major upset, Labour will resist the SNP advance in Glasgow.
|@ suemacmillan : Labour holding its majority in Glasgow is a whopper of a result. Well done @scottishlabour|
|@ JenWilliamsMEN : Ian Stewart is mayor of Salford.|
|@ baddogOTM : Labour win CHIPPING NORTON! David Cameron, Rebekah Brooks, Elisabeth Murdoch, Steve Hilton + Jeremy Clarkson now have a LABOUR council.|
With 166 of 181 councils declared, Labour have gained more than 700 council seats.
Former Libdem leader Paddy Ashdown has said the Liberal Democrats should "keep on doing" what it is doing.
Speaking to the BBC, Lord Ashdown said the party's mettle has been "tested" by the results.
"The party's mettle has been tested, it has been tested before. We should keep on doing what we're doing" he said
"It doesn't look as bad in terms of hanging onto our seats that we have got as it might have been." he added.
According to the Guardian, Ken Livingstone's team are privately conceding defeat.
They reportedly also think Boris Johnson will increase his majority.
A council candidate dressed as a penguin called Professor Pongoo has received more votes than the Liberal Democrats in Edinburgh.
Professor Pongoo (real name Mike Ferrigan) was an independent candidate who had pledged to wear the penguin outfit to every council meeting if he was elected.
From the votes counted so far, Labour are on 44%, Conservatives are on 33% and the Greens are on 8%.
The LibDems dwell at 7% while Ukip are on 5%.
Harman castigated Watson for his "hold your nose and vote for Ken" comment - and was a bit hesitant to agree that Ken had been the right candidate in the first place...we thought.
Clearly annoyed, Farage has arrived at City Hall and says that the ballot paper fail was UKIP's fault, not London Elects. But he's blaming it on UKIP's poor performance. They're currently trailing in 6th place behind Siobhan Benita
Ukip leader Nigel Farage is reportedly furious that his party's candidate for Mayor of London, Lawrence Webb, was listed on the ballot paper as Fresh Choice for London.
He says it was a 'cockup which undoubtedly cost UKIP votes' and could have helped Boris Johnson's bid for four more years at City Hall.
Also here is a round-up from HuffPost of Tory dissent following the local elections drubbing. A few themes emerge. Many Tories think House of Lords reform is a distraction - a couple but don't like gay marriage, but the overwhelming narrative is there needs to be a growth strategy.
In an update to previous news about the BNP's difficulties, it has emerged that the party has lost every single one of the 12 seats it has been defending.
|@ Kevin_Maguire : Looks like Labour will win London but lose the Mayoralty, getting most votes/seats in the Assembly but Blue Boris beats Red Ken|