Mr Cameron said his candidate was 'his own man'
Boris Johnson would "do a brilliant job" as London mayor and is "exactly the kind of leader" the capital needs, Tory leader David Cameron says.
He was "twice as charismatic, twice as energetic" as rival and current mayor Ken Livingstone, Mr Cameron said.
Mr Cameron spoke out at the official launch of Mr Johnson's campaign in Edmonton, north London.
Mr Johnson, who polls suggest is in the lead for the 1 May election, said that tackling crime was his top priority.
If elected he would set up a fund to encourage London's "wealth creators" to support voluntary sector projects tackling the city's social problems.
Mr Johnson said he believed it was possible to get more police on the streets and said creating a safer city was central to everything else that he wanted to achieve.
Earlier, he met the family of teenager Ofiyke Nmezu, who died after an assault last month. A teenager has been charged with the boy's murder.
Mr Cameron said crime was the "single biggest factor undermining our quality of life in this great city," adding 27 teenagers had been murdered last year while 11 had already been killed this year.
Mr Cameron said: "London's real problem is a lack of leadership - there's no excuse for it. One person has the job of running this city ...the mayor of London."
While the Tory leader conceded that Mr Livingstone "did do some good things" when he came to power in 2000, he said Mr Livingstone's administration was "badly run and bogged down in corruption investigations".
In contrast, the Conservatives' candidate, Boris Johnson" was "as big a figure as Ken Livingstone - and twice as charismatic".
"I was delighted when Boris decided to run for mayor because I knew he'd do a brilliant job, not just as a candidate but as exactly the kind of leader that London needs," he said.
"I don't always agree with him but I respect the fact that he's absolutely his own man. He's a proper Conservative - practical, open-minded and keen to get things done.
"And he's someone who has a properly thought-through plan to make London better."
The appearance of Mr Cameron alongside Mr Johnson comes amid signs that the Conservative leader is growing more confident of his candidate's chances of success on polling day.
Mr Johnson, an ex-magazine editor and MP for Henley, says he will leave Parliament if he wins the mayoralty.
He has promised to divert funds from the mayor's "publicity budget" to pay for 440 more police community support officers on buses and pay for more police officers to patrol railway platforms.
He also says he would encourage union bosses to agree not to strike on the London Underground and would scrap bendy buses in favour of Routemasters - which would have conductors and disabled access and be run on environmentally friendly fuel.
He would tackle the intimidation felt by Londoners on the buses by teenagers "acting up".
"I want to challenge the assumption that there is nothing cooler than the gangs," he said.
And he would target low-level disorder, such as fare evasion, which he said would make a "real difference" on crime.
Later Labour's Ken Livingstone, who is running for a third term as mayor, outlined his housing manifesto, with an appeal to first-time buyers priced out of the market.
The London mayor has plans to deliver 50,000 affordable homes in three years. Mr Johnson pledges the same level of housing by 2011.
Mr Livingstone told the BBC he had more than doubled the amount of affordable housing in London since he became mayor and had "called it right" on all the key political issues - from the congestion charge and the 2012 Olympic bid to opposing the war in Iraq.
"I think the mayor needs to get 90% of decisions right or the city's in trouble," he said.
"You need to get all the big decisions right. It's all very well Boris now saying we will broadly keep the congestion charge but you need to get it right at the time and he was opposing it."
The Lib Dem candidate, former police chief Brian Paddick says he has a proven track record in reducing crime.
As Lambeth borough commander Mr Paddick pioneered a controversial "softly softly" approach to cannabis possession - where the drug was confiscated but people were dealt with informally, not arrested.
He told the BBC that during that trial cannabis seizures rose 110% and said that it was a response to the particular problems of Lambeth at the time.
"I would not advocate going softly on drugs now, in any shape or form," he said.
Referring to the deaths of teenagers, he added: "I have advocated more police stop and search to take guns and knives off the street."
Ten candidates will take part in the London mayoral election on 1 May. Local and London Assembly elections take place on the same day.