I happened to be present on June 21, 1986, in Cobb's Hill Park, when the Topless Seven took off their shirts, and in doing so made topless legal in NYC.
Yea there is an annual march around the country (in legal states) with it as well. I think there is an organization out there that actually tries to help defend anyone who is actually cited too.
In my previous home of Vermont it's long been the case that nudity is not at all illegal - it's illegal to act lewdy in public and removing your clothing is considered a lewd action. As such walking into public naked is generally naked, but only generally, in a lot of cases folks can still make a case for your arrest if you're not really careful about the company you keep.
It's a little absurd to claim that nudity isn't illegal, but taking your clothes off is (illegally) lewd. Vermont has effectively kept nudity illegal without having it explicitly on the books.
Oh - the loophole is that you can walk out your front door naked. And taking your clothes off isn't illegal in any stated law, it's just been generally found to be a lewd action when challenged, while merely existing naked hasn't been.
It's similar in the UK, in that it's deeply contextual. Walking around naked somewhere where people are likely to take offence is dicey, but the more expected it is to encounter someone naked somewhere, the safer you are.
The specific things that makes it a crime would be intent to "cause distress, alarm or outrage", or if you are "outraging public decency".
And so e.g. a nudist beach generally does not need any particular permissions on the basis that merely signing it properly, or the place being generally known to be somewhere where people are naked makes it hard to argue it causes distress, alarm or outrage to come across someone naked there.
Of course where exactly the line goes on that is constantly changing and tricky to judge.
E.g. someone stood naked on a plint in Trafalgar Square in the middle of London and police refused to order him to cover up [1] but another naked man on the plint was told to cover up another time.
