Ken Livingstone today vowed to eject Boris Johnson from City Hall after he was selected as Labour's candidate for the London mayoral elections of 2012.
And Mr Livingstone said the mayoral vote in Olympic year will provide a springboard to take on the coalition Government and return Labour to power nationally.
He said his message to those worried by David Cameron and George Osborne's cuts was: "If you want them out, first vote Boris out."
Mr Livingstone beat former MP Oona King by a margin of more than two-to-one in the ballot, scooping 68.6% of votes to her 31.4% and leading among both party activists and affiliated union members in the capital.
His victory - a day before Labour chooses its new national leader - sets the scene for a re-run of the 2008 poll, when he was ousted by Conservative Mr Johnson after eight years as mayor.
As well as giving him an opportunity to avenge that defeat, the 2012 poll has added spice because its victor will represent London at the Olympic and Paralympic Games, which open a few weeks after the election.
Denouncing the current Mayor as a "joker", Mr Livingstone said his task was now to turn out his "disastrous administration in City Hall".
He unveiled a platform of policies for his fourth tilt at the mayoralty, including a pledge to keep public transport fares lower than they would have been under Mr Johnson, funded by the extension of the road traffic congestion charge zone into west London.
Other pledges include a victims' commissioner for those affected by crime; guaranteed neighbourhood policing; using new technology to make London a "smart city"; protecting the green belt; and encouraging emerging economic giants like China, India and Brazil to make London their preferred base for European operations.
Mr Livingstone said: "Today's decision by London Labour members signals the start of a campaign to change London for the better and to protect Londoners from the cuts of this Government that threaten to wreck lives and push us back into recession.
"The London election in 2012 will be the chance to send a message to David Cameron and George Osborne that we don't want devastating cuts to our public services, fewer jobs, and declining living standards.
"If you want them out, first vote Boris out."
Mr Livingstone predicted that Mr Johnson would be a tough opponent, who he said would be "well-financed by the rich and the powerful" and would seek to win by dividing London's outer suburbs from the less wealthy inner city.
But he said that the Mayor would be fighting on a record of cuts which have hit the suburbs as well as the centre. And he insisted that Mr Johnson would not be able to say "It's not me, guv" when the impact of cuts imposed by central government are felt by Londoners.
"Boris, your fingerprints are all over the scene of this crime," he said. "It is you and your party that damage London."
Announcing the election result on her last day in the post, Labour's acting leader Harriet Harman told Mr Livingstone: "The whole of Team Labour will be backing you, Ken.
"I have no doubt that you will win the backing and support of communities across London as you stand up for them as they are faced by the unfairness of a Tory Government and a Tory Mayor."
Ms King, who embraced Mr Livingstone after the result was announced, pledged to work for him in the 2012 campaign and urged her supporters to do the same.
The 65-year-old Labour candidate's history in London politics goes back almost 40 years to 1971, when he was elected to Lambeth Borough Council.
He went on to serve in the Greater London Council, which he led from 1981 until it was abolished by Margaret Thatcher in 1986, earning the nickname "Red Ken" for his left-wing politics.
He was MP for Brent East from 1987 to 2000, and was expelled from Labour for a period after standing as an independent in the first mayoral election in 2000.
His tenure at City Hall was marked by the introduction of the central London congestion charge and the Oyster card for public transport users.
Ms King, 42, was a Blairite Labour MP for Bethnal Green and Bow from 1997 to 2005, when she was defeated by Respect candidate George Galloway in a general election battle dominated by controversy over her support for the Iraq War.
Mr Johnson's deputy mayor for policing Kit Malthouse - a Conservative member of the London Assembly - said: "Choosing to exhume Ken Livingstone is a very odd decision.
"Granted he is a game old boy, but we had assumed that Labour would choose the future, not the past.
"I know that Boris wants to talk to Oona King about some of her new ideas, and in the meantime we look forward to hearing from Ken, Lee Jasper and the rest of the Livingstone gang."Reuse content