In a move that took the party leadership by surprise, the former GLC leader went on the offensive with a full-page newspaper advertisement announcing a "Back Ken" rally to be held next week.
The advert, in the London Evening Standard, will be followed by thousands of leaflets carrying the slogan "Let Ken Livingstone Stand" as the Brent East MP takes his message directly to Londoners and party members.
Mr Livingstone's supporters hope that the campaign will attract enough support to make it too embarrassing for Labour to block him from any party shortlist for the mayoralty.
However, the new publicity blitz was immediately attacked by Blairite loyalists who dismissed it as a "desperate, last-chance" tactic that was likely to backfire.
The rally at Central Hall, Westminster, will be backed by the comedian Jo Brand and the pop singer Billy Bragg and will focus a concerted attempt to set up a pro-Ken organisation within each of the 14 districts forming the new Greater London Authority.
The advertisement, titled "Read Ken?", uses nine of the MP's recent quotes to counter what he claims is an attempt by Millbank officials to smear him as a dangerous left-winger.
One of the quotes, taken from an open letter to Tony Blair last month, states: "There is simply no question whatever of my seeking to use the mayorship as a platform to wage political warfare against this Government." Another quote, from The Independent last November, states: "Personally I am in favour of Labour winning elections, which means selecting candidates from the widest and most representative pool, and taking the troops with you."
The campaign will be funded by public contributions, he said, and appealed for supporters to send him "the price of a pint of lager or a pack of fags each week".
Mr Livingstone stressed yesterday that he wanted to show the Prime Minister the strength of feeling among the ordinary public in the capital.
"This is not my campaign to be Mayor. This is my campaign to be allowed to stand. It is aimed at changing the one vote which really matters, which is Tony Blair's.
"It's ridiculous to think it's been nearly a year since Londoners voted in a referendum and still Labour is not getting its act together and the Tories are off and running."
Labour's strategy was to put the contest "off and off" in the hope that his campaign would self-destruct, he claimed. "I have no intention of self-destructing," he said.
A party spokesman refused to comment on Mr Livingstone's campaign, but Joan Ryan, Labour MP for Enfield North and member of the London Labour Party board, accused him of "childish" tactics. "It is not acceptable. I think the public are fed up with it. He should wait his turn," she said.
London's 70,000 Labour Party members will be asked this autumn to vote on their choice in time for the election in May 2000.
But the party leadership is still wrestling with the problem that it has no mainstream candidate who is likely to beat Mr Livingstone in a one-member, one-vote contest. A selection panel has been appointed by the party's National Executive and is expected to draw up a shortlist after the European elections in June.
Party sources have frequently made plain that the panel will block Mr Livingstone's nomination on the grounds that he has criticised party policy and failed to back the idea of a mayoralty until recently.
Gillian Shephard, the shadow Environment Secretary, said: "If they cannot trust the people of London to choose their own Labour candidate, then their talk about trusting the people is empty prattle."