Here's an excerpt from a letter DOJ sent to the Baltimore Police Department on this issue (pdf):
Because recording police officers in the public discharge of their duties is protected byThere have been some ridiculous horror stories about people being arrested and charged with crimes for recording police officers while they conducted arrests. Hopefully the DOJ letter will put an end to this foolishness and help the police recognize that more information and openness about their activities will lead to more trust between the community they are sworn "to protect and serve," not less.
the First Amendment, policies should prohibit interference with recording of police activities except in narrowly circumscribed situations. More particularly, policies should instruct officers that, except under limited circumstances, officers must not search or seize a camera or recording device without a warrant. In addition, policies should prohibit more subtle actions that may nonetheless infringe upon individuals’ First Amendment rights. Officers should be advised not to threaten, intimidate, or otherwise discourage an individual from recording police officer enforcement activities or intentionally block or obstruct cameras or recording devices.
Policies should prohibit officers from destroying recording devices or cameras and
deleting recordings or photographs under any circumstances. In addition to violating the First Amendment, police officers violate the core requirements of the Fourteenth Amendment procedural due process clause when they irrevocably deprived individuals of their recordings without first providing notice and an opportunity to object.
Hat/tip to Digby.