Brian Paddick has said he is relieved to have joined the Lib Dem ranks after a career in the "authoritarian" police.
The former Metropolitan police deputy assistant commissioner said he had been "a police officer working undercover - as an undercover Liberal Democrat".
Mr Paddick is one of three Lib Dems vying for the chance to take on Ken Livingstone in May's London elections.
Barrister Chamali Fernando and former deputy party president Fiyaz Mughal also spoke at the mayoral hustings.
Speaking at a party conference fringe event in Brighton, Mr Paddick attacked the targeting of stop and search on people from ethnic minorities, and said police officers, not community support officers should be deployed in London's difficult inner city areas.