Don't physically try to stop the police. That's going to cause trouble. But if they're doing something out of line, record it if you can. If they tell you to stop, you can claim that you have the legal right to do so. If they continue to tell you to stop, stop. But still watch if you can.

Note well: IANAL. This is not legal advice. In particular, if anyone with real knowledge wants to correct the "If they tell you to stop, you can claim that you have the legal right to do so" part, feel free.

This is the metapolice we're talking about, not the police. The metapolice have no jurisdiction over ordinary citizens. Now, if police were to interfere in lawful metapolice activity, that would be one hell of a crime.