Originally Posted by Ghosthex
Clarification: your second sentence is a little misleading. If you are on private property and an appropriate representative asks you to stop, you only need to stop long enough to walk off their private property (and ideally onto public property), at which point you're free to resume.
Also, regarding the lady who freaked out on you, you should have said tactfully and directly that you're on public property and have the right to take pictures. Any effort to interfere with that is coercion, and the police will be called if you continue.