Report it.
Time, place, reg, direction of travel etc.
Most police cars are fitted with data recorders, and will be easy to locate which one it was, who was driving etc.
Not only that; note the name and number of every officer you speak to from now on, so they can't deny it, or run from the responsibility of being competant safe drivers. No police drivers are above the law.
I assume you were lit up like an xmas tree?
Google maps location? Lighting? Other traffic? Weather/visibility?