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Boris Johnson pledged to make tackling crime in the capital his number one priority as he launched his bid to become Mayor of London.

The Tory hopeful described the killings last year of 27 teenagers in the city as "a symptom of something badly wrong in our society".

Speaking at a community centre in Edmonton, north London, Mr Johnson said if he was elected on May 1 he would take charge of the Metropolitan Police Authority.

He also announced plans to set up a mayor's fund to encourage the city's "wealth creators" to support voluntary sector projects aimed at tackling London's social problems.

"If a mayor does not give a lead, and if a mayor does not throw himself wholeheartedly into the struggle, what is the point of having a mayor," he said.

Mr Johnson said he believed it was possible to get more police on the streets of London.

Creating a safer city was central to everything else that he wanted to achieve, he said.

He also denounced the "waste and cronyism" under Mayor Ken Livingstone, accusing him of running a "superannuated Marxist cabal".

Conservative leader David Cameron attended the launch and hailed Mr Johnson as "a man who is as big a figure as Ken Livingstone - and twice as charismatic; a man who is just as determined as Ken Livingstone - and twice as energetic".

He said: "He's someone who has a properly thought-through plan to make London better."

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