Take a risk to tackle thugs - Boris

Despite fear of violent crime running high in cities after a series of fatal attacks, the Tory MP for Henley said the chances of getting stabbed while challenging troublemakers were "microscopic".

However, police in the capital insisted that people should call them if they saw criminal or anti-social behaviour, rather than risk "escalating" the situation.

Mr Johnson - who officially launches his campaign to become the Conservative mayoral candidate on Monday - said: "What drives me mad is when you see some kids behaving badly at the top of a double-decker bus, but no one has the guts to intervene because they're scared.

"If one person speaks up, they are seen as brave -- but everyone else keeps quiet. What if everyone together on that bus were to intervene? We need a bit more willingness."

Asking in an interview for the News of the World whether people should be worried about getting stabbed, the Tory front-bencher replied: "Your chances of getting stabbed are very small. You know and I know that your chances are microscopic."

He said it drove him "wild" that individuals who did step in often got into trouble with the law themselves.

Current Mayor Ken Livingstone said Mr Johnson failed to address the real issues for tackling crime in London.

"The central issue to fight crime in London is to increase police numbers and provide the Met with adequate resources," he added.

Last month 47-year-old father of three Garry Newlove died after allegedly trying to remonstrate with teenage vandals near his home in Cheshire.