It depends on the distance, whether they're facing towards the camera, whether they're identifiable, whether they give consent and where the photo is being taken. I think you'll need some degree of discretion to figure out what's appropriate, but all of these factors come into play.
However there is another factor, and that's if the person in the photo forfeits the right to be photographed. For example if they're engaging in criminal activity, are wanted by the police, are the police engaging in unethical behaviour or are part of a significant historical moment (granted, the last one can't really be determined until after the fact).