I would refuse initially to move on by the police if I was in my legal right to be there and to take the shot. Pointing the above out and saying that to satisfy someone else they are infringing on my rights to do what I am doing. Of course if this does not work move you will lose. First asking for the officers name and badge number (in the UK the police legally have to give this if asked) and then writing to the relevant person of my dissatisfaction in the handling by the police. And when this is sorted via letters go back and take the shot taking said letters with me in case of reoccurrence of the problem.