Former top policeman Brian Paddick was named as the Liberal Democrat candidate for London mayor today.
Mr Paddick, a former Metropolitan Police deputy assistant commissioner, was selected after a winning a ballot of the party's London members.
He said "less crime, better transport, cleaner air and fewer Londoners living in poverty" were crucial for London.
Mr Paddick said: "I am enormously proud to be announced today as the Liberal Democrat candidate for London mayor.
"For too long Londoners have been denied a serious debate on the future of their city.
"Less crime, better transport, cleaner air and fewer Londoners living in poverty are all crucial in making sure that London remains one of the most exciting cities in the world.
"I strongly believe that the position of mayor can be a powerful source for good. During the coming months I will be explaining to people throughout London why the time has come for a change."
Mr Paddick, who quit the Metropolitan Police earlier this year, beat competition from two rivals for the Lib Dem candidacy - 28-year-old barrister Chamali Fernando and former party deputy president and Haringey councillor Fiyaz Mughal.
The scene is now set for possibly the most high-profile contest since the creation of the post of the directly-elected Mayor of London in 2000, with all three parties fielding well-known candidates.
Mr Paddick stands in next year's contest against Labour's sitting mayor Ken Livingstone and the Conservatives' Boris Johnson.
Acting leader of the Liberal Democrats, Vince Cable, said: "I am delighted that the Liberal Democrats are able to put forward such an excellent candidate.
"Brian is a lifelong Londoner with the high-level experience needed to be London mayor.
"London is one of the greatest cities in the world and in Brian Paddick Londoners finally have the serious candidate they deserve."
Before his resignation from the Met, 49-year-old Mr Paddick was Britain's highest-ranking openly gay police officer.
A London-born Oxford University graduate who worked his way up through the ranks, he was a sergeant on the front line in the Brixton riots of 1981 and returned to the area when he became the Met's Commander in Lambeth in 2000.
He sparked controversy by ordering officers in Brixton not to arrest or charge people found in possession of cannabis some years before the drug was downgraded from class B to class C.
In his manifesto Mr Paddick promises to overhaul London's congestion charge to target "the chief executive in his chauffeur-driven car instead of the delivery driver who keeps the capital supplied", as well as recruiting more police and converting the Tube to run off renewable energy.Reuse content