At half-term I was on a bus with my five-year-old daughter. She had wanted to see Buckingham Palace, so we saluted the Queen and walked back up The Mall to get a bus home from Trafalgar Square. The bus was packed, around us were a friendly posse of young people of all races, bantering, cracking jokes, smiling. I felt happy - when the capital's mad melting pot works, it's great.

Suddenly there was a kerfuffle among the packed-in bodies by the doors. From the shouts, it seemed that a middle-aged white guy had stepped on a young black man's foot. The bus stopped and as the doors opened, the young guy started punching the older man in the head. He wrestled him off the bus, kicked him to the ground and left him there. Calmly he got back on the bus.

Silence fell. My daughter looked at me anxiously. I hugged her and whispered not to worry, but when I looked up I inadvertently caught the thug's eye. "What you f****** looking at?" he yelled. I cast my eyes down quickly, glad that there were many bodies rammed in between me and him. No one moved, said or did anything. We all tried, desperately, to mind our own business. The bus continued on. About three stops later he got off. The chatter resumed as if nothing had happened.

I don't think that bus was full of cowards. We were just realists. We read the papers, we know the story about the man who got stabbed on a bus for asking a young guy to stop throwing chips at his girlfriend. The reality of the level of violence among a particular subsection of society is such that sometimes it's just not safe to intervene any more. I didn't want to be a hero if death could be the consequence. I just wanted to get home from a day out with my daughter with both of us in one piece. Like everyone else on that bus.

Afterwards I felt ashamed and found myself murmuring, "All it takes for evil to triumph is for good men to do nothing."..

Boris Johnson says that if he becomes mayor of London he is determined to tackle the violence: he wants conductors back on buses (who might have intervened on my bus, I suppose) and says he'll take free travel away from teenagers who abuse that right. That's all a step in the right direction - but it's not going to put the genie back in the bottle.

In New York, through tough, plentiful policing, Rudy Giuliani reclaimed the subway and public spaces for ordinary people. Boris, can you do that for London?

I'm in my twenties and sixteen stone of boxer. Could I have intervened? Yes. Would I? No.

And why not: It's because I can't risk the little dear being armed with a knife. Because then I might have to do some serious damage to him very quickly to avoid being stabbed. And though I probably could, I wouldn't want to. Because I don't trust the law. I can't risk losing my job, house, and all the things I've built up to a pc police force.

Show me a legal system that will stand up for me, and I'll intervene. Until then, enjoy the fruits of a socialist utopia.

-- MD, London

All those who have undeserved faith in the police need to get a grip. While I was in Amsterdam, I was robbed with a knife to my neck a few hundred yards from a police station, and when I went to tell them about it, they told me I couldn't report the crime until the next day. THE NEXT DAY! I don't believe this is just a function of them being Dutch, and the criminals know they can get away with these things, and that's why this type of stuff continues.

The woman was with her five year old daughter. How many people would risk even potential death in front of their very young child? Give the woman a break.

-- Becky, Santa Cruz, CA

"I don't think the bus was full of cowards." No, but the bus driver should have stopped the bus and the police should have been called. Until the law is taken seriously things will only get worse.

I have seen incidents on American public transport, much less serious than the above, and, without hesitation, the police are called to investigate.

-- Eddie Pratt, San Francisco, USA