However, if you get out and about with a camera with any regularity, sooner or later you're likely to run into an overzealous secrity guard or a woefully underinformed police officer who will tell you that what you're doing is illegal. Often these people will act in an arrogant and intimdating manner, and they usually don't expect the photographers they confront to be very well informed about the legalities of what they are doing.
These confrontations have often resulted in photographers being intimdated to the point that they will comply with an order to erase the pictures they've taken. Don't let this happen to you.
Attorney Bert P. Krages has put together a pocket-sized,
printable PDF file that spells out generally what rights you have as a photographer working in public spaces. He also links to a version for photographers in the U.K.
Go read, download and print some out to keep in your wallet or camera bag.