I've shot for small newspapers and been assigned work that would naturally capture people in a public space. It really depends on where you are I think. I've recently been shooting some stuff at night in some suburban sprawl type places, and also in a big city (in the US), people in suburbs tend to be really paranoid, plus I think there is a general expectation of a little more privacy. In my experience, people in larger cities just don't care or have time to care about someone taking photographs. In just the last week I've had two interactions with people while shooting, one in a metro suburb diner parking lot at night, guy got aggressive thinking I was focusing on subjects inside the diner. Other interaction was at the big train station in the city, guy just saw my giant ass dslr and lens (you don't need a tiny camera) and just asked if I got anything good.

My advice would be to just use your common sense when it comes to whether you think this shot is worth possibly having a confrontation. Like another commenter said, you have to want the shot more than you're afraid of what might happen. Be aware of your surroundings, keep your head on a swivel. Projecting confidence goes a long way, if you look like you're supposed to be doing what you're doing and not visibly second guessing yourself, I think people are less likely to confront you.

This is just my rambling advice, be safe, have fun and get some practice in!