Back in the '80s when I lived in San Diego, the SDPD committed so many egregious and horrendous crimes against the public (refer to examples like the Sagon Penn and Kara Knott cases, for two of many, many examples) that folks organized, monitored the police band radios and maintained contacts near wherever the cops pulled someone over in our communities so they could be contacted to quickly assemble as witnesses to how the cops treated those they stopped.  Worked, too.  Evildoers hateth the light.

I saw example after example where the cops, suddenly surrounded by a crowd of witnesses, stopped beating victims' and released 'em without charges.  My son was one of 'em.  His crime was driving a car considered too nice to be driven by a 'person of color'.  

As I recall San Diegans were still monitoring the cops until about 5-6 years ago.  All probably "illegal" now, of course--we're not supposed to be able to defend ourselves even passively from the "minions of the state."