Livingstone, who was mayor for eight years before being ousted in 2008 by the former Conservative MP, said he would fight against government spending cuts and push for lower public transport fares.
He would also look to restore the western extension of the London congestion charge, he said during his acceptance speech.
The ex-mayor said he would work to engage with the leading emerging economies of China, India and Brazil, to persuade them that London was their preferred base of operations in Europe, if he won.
Both candidates are likely to campaign hard on transport, reducing crime and improving the urban environment, but electioneering for the 5 million votes are set to be overshadowed by government spending cuts.
The Conservative-Liberal Democrat coalition plans to cut about 25 percent from many department budgets in an attempt to tackle a record peacetime budget deficit.
"I want to unite Londoners around this message: the London election in 2012 is their chance to send a message to (Prime Minister) Cameron and (finance minister) Osborne that we don't want these devastating cuts to our public services, fewer jobs and declining living standards," Livingstone said.
"We will stand up for London and to do that we need to change the mayor. If you want to get them out you start by getting out Boris Johnson."
The media has suggested that Johnson, 46, may be forced to distance himself from his colleagues in central government if he is to win a second term.
Livingstone, who stood as an independent candidate in his first election campaign before being readmitted to the Labour Party in 2004, admitted Johnson would be a "tough opponent."
But the 65-year-old said his campaign against former Labour MP Oona King for the party's nomination would provide a "positive base on which we will build to get rid of Boris Johnson" in 2012.
Livingstone received nearly 70 percent of the Labour vote.
The Liberal Democrats will announce their candidate on December 4.
