Write down everything you remember.
Police misconduct cannot be challenged on the street. Don't resist or threaten to file a complaint.
Write down everything you remember, including officers' badge and patrol car numbers, which agency the officers were from, and any other details. Get the contact information from witnesses. If you re injured, take photographs of your injuries (but seek medical attention first).
It can be overwhelming and stressful to take on an agency as large as the police on your own. They will do everything to protect their own, even if they have broken the law.
Contact a lawyer to discuss your case.