There is also the thinking.... 'Nobody would be daft enough to steal a bike in broad daylight on a busy street, it must be his and his lock is broken or something'....

Feba... I agree, I would call the police.. and yes get a shot with my cellphone camera, and even hang about to tell the cops which way they went, but I would in no way challenge them personally whilst they are to all effect armed with dangerous weapons. I speak with the voice of bitter experiance, although not with a situation like this. But I once played the good samaritan and ended up having surgerey on my face. Let us also not forget he may not be stealing the bike to use, but to feed a habit... and they may be very desperate to feed that habit. Desperate people can be dangerous.