Apparantly both version are in use according to Google Books Ngram. However, the use of in public far more common than at public.
I wonder if saying at public is wrong or non-standart English even though it is in use.
For example :
Smoking in/at public should be forbidden.
Best Answer
I don't suppose you clicked on the links below the ngram that show the actual hits?
If you peruse the found instances, you can see that "at public" is used when public functions as an adjective, in phrases like public schools, public meetings, or public parks.
So, you would not say:
but you might say: