In his speech at City Hall, the Labour candidate says he will not run in another election.
He added: This defeat is the worst.
Alex Forrest writes:
While we wait here at City Hall for all the London votes to be counted, I've already seen one mayoral candidate in our press centre.
Siobhan Benita the independent is here and she seems pretty happy about how things are going.
Boris Johnson still seems to be in the lead, though according to the live count chart on londonelects.org Ken Livingstone does seem to be making a comeback.
I've just spent the past hour in Hammersmith on the campaign trail with Ken Livingstone.
He was met by supporters waving placards saying "24 hours to cut fares". He got on his soap box to tell anyone who'd listen this was London's last chance.
However when it came to my interview with him he was clearly irritated.
He dismissed as irrelevant the fact that bookmaker paddypower is already paying out for Boris Johnson.
And he said those from within the Labour party who've been less than supportive of his campaign are, and I paraphrase, has-beens.
Mr Livingstone also appears to think that it's the media who have ruined his campaign because we have been too negative.
I am waiting to interview Boris Johnson now. He's got his own problems to deal with and is clearly distancing himself from the Tory party at the moment. Let's see what he has to say.
A Populus poll for The Times shows - in the first round of voting, Mr Johnson has 46 percent, while Ken Livingstone has 34 percent.
The Green party candidate Jenny Jones is in third place with 6 percent.
Brian Paddick the Liberal Democrat candidate has five percent, along with Siobhan Benita the Independent candidate.
UKIP's Lawrence Webb polls three percent, while the BNP's Carlos Cortiglia has one percent.
A new YouGov poll for the Evening Standard has found that the London mayoral race is once again neck-and-neck. Labour's Ken Livingstone has narrowed the lead to just 2%, once second preference votes are taken into account. The current mayor Boris Johnson leads by 51% to 49%.
The Labour mayoral candidate Ken Livingstone has insisted that his plan to cut transport fares in London by 7% is affordable. Transport for London has said the cut could threaten investment but Mr Livingstone thinks it will boost the number of passengers.
Our Political Correspondent Simon Harris writes:
Apprentice star and business tycoon Lord Sugar has fired a potentially damaging shot at Ken Livingstone's campaign to be London's next mayor.
Lord Sugar told his 1.8m Twitter followers: "I seriously suggest NO ONE votes for Livingstone in the Mayoral elections."
The surprise intervention by a Labour peer will be regarded as extremely unhelpful by Livingstone's campaign team.
Labour's candidate has been dogged by controversy over his tax affairs.
Recent opinion polls show him lagging behind his Conservative rival Boris Johnson.
The Livingstone campaign publicly played down Lord Sugar's comment, insisting the two men have a long history of not being friends.
A full list of the candidates running in the mayoral elections can be found here.
A senior Labour source has indicated that Lord Sugar will not face disciplinary action over his tweets.
The Labour peer took to the social networking site earlier today, to urge people not to back Ken Livingstone in the London mayoral elections.
The source said: "There is a long history of Lord Sugar and Ken Livingstone not being the best of friends".
"Lord Sugar was not recommending people vote for any of Ken's opponents.
"He is a Labour peer but Lord Sugar's views are very much his own."
A full list of the candidates running in the election can be found here.
Boris Johnson's campaign team is claiming that his Labour rival has made at least £2.7bn worth of unfunded promises during his mayoral election campaign.
The Conservative campaign says that Ken Livingstone will have to increase council tax and increase the congestion charge in order to meet his promises.
A spokesman for Boris Johnson said: “Ken Livingstone says one thing but does another. He is making promises to Londoners which he knows he can’t keep.
“He has made £2.7billion worth of promises at a time when public finances are the tightest they have been for decades.
“There is only one way he can keep his promises – and that is by continuing his record of hiking up council tax and increasing or expanding the congestion charge zone at a time when Londoners are struggling to make ends meet."
The full list of candidates standing for mayor is as follows:
Siobhan Benita (Independent), Carlos Cortiglia (BNP), Boris Johnson (Conservative), Jenny Jones (Green), Ken Livingstone (Labour), Brian Paddick (Liberal Democrat), Lawrence James Webb (UKIP).
Boris Johnson has edged ahead of Ken Livingstone in the race to become Mayor.
An exclusive poll, commissioned by London Tonight, the Evening Standard and LBC, suggests the current Mayor is now six points ahead of his Labour rival.
Daisy Gray reports.