Georgetown law professor Paul Butler explains that you have a constitutional right to record video of the police in a public place, so the cops in Ferguson broke the law when they stopped journalists and civilians from doing so:
The law is simple, and it is entirely on the side of the citizen photographers. The First Amendment of the U.S. Constitution protects the right of anyone to record police in a public place. The police can place reasonable restrictions on photographers by, for example, not allowing them to enter a crime scene. But they cannot stop people from standing on the street and filming them while they make arrests, detain suspects, or otherwise enforce the law.
Read Butler's full piece here.
If the police see you filming, they cannot force you to turn over your camera. They cannot make you delete what you have filmed. Of course, they can ask you to do any of these things — and the police are very good at making requests sound like orders. But all you have to do is say something like “Officer, I refuse to consent to you to look at my photos.” Then you have the constitutional right to be left alone.
It takes guts to record the police, even if it is perfectly legal. As I often tell my law students, the Bill of Rights is not for wimps. But think of it as an act of patriotism. The United States Court of Appeals for the First Circuit affirmed, in 2011, the right to video-record the police. The Court stated “Gathering information about government officials in a form that can readily be disseminated to others serves a cardinal First Amendment interest in protecting and promoting ‘the free discussion of governmental affairs.’”
Police should support these efforts. Cameras improve working conditions for the majority of police officers who are hard-working and law-abiding. In jurisdictions where police cars are equipped with dashboard cameras, police misconduct complaints have gone down – along with the taxpayer expenditures to settle them.