If you photograph the police doing something out of line, snap that pic and take the ACLU’s advice.

If you are stopped or detained for taking photographs:

• Always remain polite and never physically resist a police officer.
• If stopped for photography, the right question to ask is, “am I free to go?” If the officer says no, then you are being detained, something that under the law an officer cannot do without reasonable suspicion that you have or are about to commit a crime or are in the process of doing so. Until you ask to leave, your being stopped is considered voluntary under the law and is legal.
• If you are detained, politely ask what crime you are suspected of committing, and remind the officer that taking photographs is your right under the First Amendment and does not constitute reasonable suspicion of criminal activity.

via Know Your Rights: Photographers | American Civil Liberties Union.