Police are policing the scene.
Police are at the scene.
Policing the scene makes no sense. It is much better to use, at the scene. If they are there, of course they are "policing," the scene.
exists e1 x1 x2.(n_scene(x2) & Theme(e1,x2) & Actor(e1,x1) & v_police(e1) & n_police(x1))
exists p1 x1 x2.(n_scene(x2) & at(x1,x2) & n_police(x1))