The law allows one to record as long as you are not infringing on another's privacy. Typically, recording anything in the public view is acceptable. Aside from whether it is legal to do so, the police may not appreciate you recording their actions. I have had clients who were threatened by police officers for engaging in this type of behavior. Your actions cannot hamper the officers investigation or arrest of you either. You could theoretically be charged with Obstruction Justice if your recording action somehow prevents the police from doing their job. Your actions cannot cause problems either. An example would be someone who stops a car on a busy stretch of highway to video tape the arrest of another citizen.
Generally you may record anything that goes on in public as long as you are not violating any laws, i.e. stopped in the middle of traffic to record the arrest of an individual.