Ken Livingstone has 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.
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Mr Livingstone said: "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."
