This question comes up a lot. Here is my summary. Take it for what it's worth (which means I'm not a lawyer).
You can pretty much take photographs of anything or anyone visible from a public place while you're in a public place. Public places typically include parks, roads, sidewalks, etc. But there are sometimes restrictions if you block access, create a safety problem, are engaged in taking pictures for commercial purposes (eg. parks may require a permit), you're trying to photograph classified facilities, etc.
Even if you're in a public place, you can never take photographs of people who have a reasonable expectation privacy. You can't, for example, take a picture of someone through a window when they're in their home. Even if you are shooting from the public sidewalk. Ditto for photographing someone entering their PIN at an ATM. You get the picture.
And even if you are in a public place, and they're not in private, don't be a jerk. Don't be intimidating, threatening, or unpleasant. If they ask you to stop, stop. The law may not require it, but remember that anyone can sue. Whether they win or not is a different thing. But do you really want to spend time with lawyers?
If you're on private property (in other words, not in a public place), the property owner can restrict your ability to take pictures on their property (eg. a building), but they cannot prevent you from photographing that building from public property. A manager at a Starbucks might ask you to not take pictures inside the store, but they can't stop you from stepping outside to the sidewalk and taking pictures of the building.
No one, other than a law officer making an arrest or executing a warrant or subpoena has the right or authority to confiscate your film, memory cards, camera, or equipment. They also can't make you erase any pictures either. If they're a security guard, they're not a law officer and have nothing even vaguely resembling that kind of authority.
If you get hassled, it's likely to be by a security guard or employee on a power trip. Get their name and number, and the name of their employer. They're probably very badly informed about your rights and their authority. It's a very good bet that you will not win the argument. And they might be right. There might be some little known law or regulation that says you can't take pictures inside a Post Office. Which, ironically, is NOT a public place.
If someone asks you what you're doing, that doesn't mean they're bad people. They just don't understand. They might be responding to a call from a concerned citizen and they have to ask questions. Maybe you're acting in a way that's "suspicious." So make yourself less suspicious by explaining it to them. Try to deal with these situations by seeing things from their perspective.
Finally, here are two important resources from Bert P Krages II. He's an attorney specializing in this stuff and has written a nice, one page summary that you can tuck in your camera bag called The Photographer's Right. I also strongly recommend his book, Legal Handbook for Photographers. It's got a lot more important information that you'll find useful.
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