IMHO, you'd be asking for trouble walking your chicken on a leash "around the city". Even a dog on on a leash can/would pull away from his handler to get to your chicken. There are other predators that may not be able to resist this strange, scrumptious sight, either. You and your chicken would be at risk for harm, I think.
I understand the interest in training your chickens... mine are trained for Animal Assisted Therapy. They are used at schools and nursing homes, all the time.
People are interested in the chickens! Many young people and adults have never touched a chicken - many elderly people haven't been around chickens since their youth. Chickens are wonderful therapy animals, because they are unusual, social, and gentle (if trained properly).
If trained properly, and in a proper setting, you won't need a leash. Walking on a leash down a city street is not the proper place for a chicken for many reasons, IMHO.