You should photograph the police, especially when they arresting someone or otherwise using force.
We had a case last year here in Stockholm when a young man filmed a questionable arrest, and then the police (dressed in civil clothes) turned on the guy with the camera phone recording the whole thing. He was threatened and forced to erase the memory of the camera. Later, the film was restored and the whole thing turned into a public scandal. Don't remember thought if the policemen were punished.