Lib Dem mayoral hopeful Brian Paddick has launched a personal attack on his Labour rival Ken Livingstone, calling him a "nasty little man".
Mr Paddick told the Evening Standard the mayor was "very unpleasant" and treated all critics with "contempt".
The former police commander said Tory candidate Boris Johnson was "somewhat eccentric" but "harmless".
Mr Livingstone, who is running for a third term as mayor, claims London is more successful than ever under him.
Mr Paddick, the former Lambeth police commander who pioneered a "softly softly" approach to cannabis, told the newspaper Mr Livingstone had tried to set up a "socialist republic" at City Hall and accused him of cronyism.
He said the mayor relied on a close circle of left wing advisers and had stopped listening to Londoners.
He added: "Do you want somebody who is a really nasty little man in the shape of Ken Livingstone, very unpleasant and rather nasty, or somebody who just appears to be somewhat eccentric but otherwise really harmless as an individual, except I wouldn't trust him to run anything for me?"
He said he had considered a Conservative approach to run for them but "on principle" could not stand for "what the Conservatives stand for".
Mr Paddick, who has put himself forward as the "serious candidate" for London mayor, has pledged to act on voters' priorities not "personal agendas".