[DennisP]

You wouldn't want to legalize lynch mobs. But maybe there's a happy medium. Making it clear that citizens are allowed to take videos of cops might be a good start.

For actual punishment, force isn't necessarily needed. A lot of societies manage it by social pressure, with "shunning" being an extreme version. Pressure for conformity already exists regarding things which are not illegal, so I'm not convinced that it makes things worse to rely on it more for things which are currently illegal.

One of my anthropology professors used to say there are two ways to keep order in society: by force, and by ideology, and the second is a lot cheaper.

[sliverstorm]

It's rare but you can perform a citizen's arrest

[amalcon]

It's also actively discouraged by the police for a number of reasons. Some of those reasons are even good: chain of custody concerns for any evidence on the suspect's person, danger to the untrained person making the arrest, danger to the suspect, and the possibility that the suspect will sue.