Boris vows to tackle violent crime

Mr Johnson, who defeated Labour's Ken Livingstone by six points to become mayor, said that the number one issue facing London was the "tragically self-destructive" behaviour of some of its young children.

He also pledged to make good on campaign promises to build affordable homes, protect gardens against development, improve bus services and crack down on minor crime on public transport.

The formal signing ceremony at City Hall came just hours after a 15-year-old boy was found stabbed to death in a stairwell in south London - the latest victim of a wave of knife-crime among young people in the capital.

Describing the boy's death as "tragic", Mr Johnson said: "I think this problem of kids growing up without boundaries and getting lost in tragic and self-destructive choices is the number one issue we face in this city.

"It is the job of me as Mayor to lead the fightback against it and that is what I want to do and I hope you will all join me in doing it."

Without giving full details of his intentions, Mr Johnson said he would "reform and improve" the congestion charge on drivers entering central London, including by making it possible for motorists to pay their charge on account at the end of the month.

He said he would hold discussions shortly on how to clamp down on disorder on buses and trains, which he said was the single policy which most clearly resonated with the public during his election campaign.

"We are going to double the size of the safer transport teams to get people the sense of safety and security on the buses that they deserve," he said. "I think there is a vital necessity to drive out so-called minor crime and disorder as a way of driving out more serious crime."

Mr Johnson was attacked during the mayoral campaign as a divisive figure who would be unable to represent London's large ethnic minority population. But he insisted that he wants to be a unifying figure, bringing representatives of all the city's diverse communities into his administration.