Boris Johnson, the new mayor of London, made fighting crime his top priority as he officially took over his new role.

“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,” he said after signing the declaration of acceptance as mayor.

Johnson, who triumphed over Ken Livingstone by six points in Thursday’s poll, said the “tragically self-destructive” behaviour of some of the city’s children was its number one issue.

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 incident in a wave of knife-crime among young people in the capital.

Johnson said the boy’s death was tragic. “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.”

Without giving full details of his intentions, Johnson said he would “reform and improve” the congestion charge on drivers entering central London, including 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 policy that 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.” 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. “I want, insofar as it is possible, to work with everybody from across all communities in London.

“It is vital for a Mayor of London to bring people together and to unite London and to serve every community in London impartially and to make sure we have in my administration representatives of all communities, which is what we will have.” The election of London’s first Conservative Mayor is expected to lead to a turnover of senior advisers at City Hall, and Johnson said he would name some of his key aides over the next few days.

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