Quick note... many years ago, I started to do just this (in the U.S.), and was totally surprised to learn how much of what we consider "public space" is actually private property. Let alone getting moved along by the cops, I was kicked out of several open plazas in downtown areas by private security guys. It actually became fun sport. I even tried to busk in a couple of large shopping malls and would always relish the moment when I could spot a couple of rent-a-cops hovering in the perifery trying to decide what to do with me. I was relatively harmless, compared to some of the rough company they were used to moving along, so they were almost always pretty decent about the whole thing.

And, if you are in a sanctioned area (I'm sure the other threads cover this in great detail), you want to be aware/careful about the etiquette of claiming a space and territorial issues. Fisherman's Wharf in San Francisco, the Boston Common, many of the really popular touristy areas... all have well established pecking order and commonly accepted ground rules for their activities.

Good luck, have fun. And have at least a few bucks in your pocket when you start out, or you'll get busted for vagrancy. If you get whisked away in handcuffs, just keep proclaiming loudly "I am NOT resisting arrest. I am NOT resisiting arrest," and then start singing "We Shall Overcome." Great way to spend an afternoon.