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."