Linda mentioned in yesterday's comments how hard it is to sketch in public!
I used to tell my students on the Drawing for Non-Artists course that it is a good idea to sit with your back against a wall or fence or hedge. People are less likely to approach you and you can at least see them coming. And although you might enjoy folks chatting to you, if you don't want to talk, do not make eye contact and try just to keep saying Mmmm and continue drawing - they usually take the hint after a minute or so.
One student sketching in the centre of York where you can easily find a little crowd gathered around you if you are drawing, got fed up wrote WORKING. DO NOT DISTURB. in large letters at the top of his drawing board!