Boris Johnson inferred Ken Livingstone was a "newt fancying, tax dodging, bendy bus fetishist" as he launched his mayoral re-election campaign.
The colourful attack was made just as it emerged that Labour's candidate, a critic of tax avoidance, had been paid, legally, through a company instead of being taxed at source.
The Mayor of London accused his predecessor of being a "hypocrite" as well as a "glove puppet" for trade unions.
Addressing the Conservative Party spring forum, Mr Johnson said: "I'm afraid on May 3 I do see real risks for this city and this country.
"I see a risk that City Hall will be recaptured by a bunch of semi-reformed Trotskyist, car-hating, (Venezuelan President) Hugo Chavez idolising, newt fancying, tax dodging, bank bashing hypocrites and bendy bus fetishists."
Johnson also launched a defence of City workers, telling activists "It's about time we stopped abusing them."
He drew attention to the 600,000 employees who depended on the financial sector in London, many on "modest" incomes.
Describing himself as "man who has built half a bridge" he said he could see what needed to be done to complete his "modernisation" programme.
I feel like a man who has built half a bridge, or half a cable car - a massive, privately sponsored cable car. " he told the packed hall.
I can see the other side. I can see what needs to be done."
A spokesperson for Ken Livingstone said: "It sounds like the high-fares mayor has finally lost it. Not only has he not built a bridge, most Londoners don't know what he has done at all.
"While Boris Johnson was hanging out the Prime Minister who wants to destroy the NHS, Ken was campaigning in outer London for a fares cut.
Meanwhile Johnson unveiled his nine point plan for re-election, which includes council tax freezes, job creation, putting more police on the beat and cutting Tube delays.
He also pledged to secure a better deal for London from No 10, something that has already seen the mayor cross swords with Downing Street.
Earlier on Saturday, David Cameron urged party members to help get Mr Johnson re-elected.
He said: "He's defended the economic interests of London at every turn - believe me, I know.
"He's a brilliant mayor of the best city on earth so let's make sure that Boris gets another term.
"And to anyone wondering about the best thing to say on the doorstep, I give you just two words Ken Livingstone.
"We cannot let that man back running our capital city again."
It comes as Mr Livingstone, the former Labour mayor who is vying to take back the job, launched his "Fare Deal" soap box pledging to cut fares by 7% by October 7.
A campaign spokesman said: "Ken's Fare Deal fare cut is clearly shown to be the most popular policy in the election.
"As we enter the next phase of the campaign Ken will take his red soapbox with his Fare Deal message to town centres all over London, starting in the battleground of Havering."
"Boris Johnson is stuck fighting the stale arguments of the last election, when Londoners are crying out for action now to cut fares and ease the squeeze on their living costs.
"Boris Johnson has hit people hard with soaring annual above inflation fare rises that have seen a bus fare rise by half, whilst cutting police numbers at a time of rising crime.
"Ken is focused on meeting Londoners' pressing needs now: there is a clear choice on offer in May, between Boris Johnson's high fares policy and Ken's fares cut, which will make the average Londoner £1,000 better off.
"Under Ken we had real action - cutting congestion, increasing police, sharply falling crime, free travel for school kids, winning the Olympics, securing transport investment like Crossrail, and licensing mini-cabs, and he will use his proven ability to do the job to make a real difference to Londoners by cutting their fares."