"Hundreds of young people have marched through north London to the place 
where 16-year-old Ben Kinsella was stabbed to death to protest against 
knife crime."

Having lived and plyed in the areas where these crimes are happening, I 
have some bad news for these people.  The gangs that use knives and the 
like, won't give a damn.  They see themselves as a class apart and will 
continue to do whatever they like.

It will take momentous move from within to change this attitude. 
Parents will need to inform on their children and friends report on 
friends and the offenders (nearly always children) need a punishment 
that they will fear.