"Bring along a camera next time and take pictures of innocent things where everyone can see you, right away, everyone will think you are a photographer...
Hélas, that is not allways true.
Once last may, I was held by police on the street in Amsterdam (NL), after (very visibly) taking 2 pictures of a (partially closed) post office. The officer said that i was suspicious because i "was taking photos of a bank office". After checking my identity and checking my name with the local police, he let me go, so i was not arrested or taken to the police station.