You should consult with an Attorney who is very familiar with U.S. Supreme Court cases and Constitutional Law. You were practicing a right of Political Free Speech, and I believe, should not have been arrested.
However, there are exceptions to every rule. So, you really need to consult the proper Attorney to see whether you have a cause of action against the store, and the police department.
And, like everything else when we wish to exercise our rights under the Constitution, or to right a wrong, it's going to cost you big bucks to do this, because it is rare for a Constitutional Attorney to accept Pro Bono work, or to work on a Contingency basis.
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