i wondered when something like this would happen in the USA

During the Anti-G8 protests in San Francisco, a police officer who attempted to arrest a protester was badly beaten.

Here’s the story (as told by IndyMedia SF):

the cop was driving in his cruiser after a group of splinter protesters who were shooting off fireworks, putting barricades in the street
and breaking windows.
He then ran over one of the large styrofoam banners people were carrying.
Somehow, this affected the forward movement of his car as the wheels spun.
Then, some kind of projectile (maybe a firework) shot at his windshield.
Infuriated, the cop leaped out of the car and started swinging.
Unfortunately for him, this aggression was a bad move and he was physically beaten.

A legal observer then informed other officers nearby who were beating protesters that there was an officer down.
This then became their top priority and a bunch more cops showed up on the scene.
These cops began swinging and attacking anyone they could see (even the legal observer who told them about the injured officer).
They then cleared the scene and called for an ambulance.

QUID PRO QUO…bro.
QUID PRO QUO.