Ken Livingstone has launched his campaign to be Labour's 2012 London mayoral candidate, criticising current mayor Boris Johnson's transport policy.
Mr Livingstone was the city's first elected mayor, between 2000 and 2008, before being ousted by the Mr Johnson.
Former MP Oona King is the only other person officially to put their name forward for the Labour candidacy.
Mr Livingstone launched his campaign in Croydon in south London. The party will choose its candidate on 22 September.
He said his top priority would be holding down public transport fares.
"Under Boris Johnson, a single bus fare by Oyster has risen by a staggering one third, as has the price of a weekly bus pass," Mr Livingstone said.
"Instead of wasteful projects, the obsession with academy schools, the zany floating airport in the Thames Estuary, we must concentrate on defending public services and holding down fares."
Mr Livingstone said he wanted to be mayor "for one overriding reason".
"If I am elected my focus will be to do everything I can to protect Londoners from the recession and the effects of the government's policies," he said.
"A government removing billions from the economy and planning cuts on a scale that Britain has not seen for decades, means the mayor's priority must be to protect Londoners."
Mr Livingstone said his mayoral campaign would be led by Tottenham MP David Lammy whom he has known for more than 10 years.
Mr Lammy said: "I saw what a recession can mean for Londoners during the 1980s. There were riots just a few roads away from our family home.
"London needs a political heavyweight to stand up to this government.
"Ken is that figure. He knows city government and coalition politics inside out, he has been through countless Whitehall spending rounds and he has London in his soul."
Labour mayoral candidate hopefuls have until 18 June to put their names forward and will then be whittled down to a shortlist by a panel of national and London party representatives on 24 June before a series of hustings.
An electoral college, made up half-and-half of votes by London party members and members of affiliated organisations, will then pick the candidate.
The winner will be announced the day before the party reveals who has won the race to succeed Gordon Brown as national party leader.