Here are the photos that I took of LAPD undercovers at the 4/17/2010 Nazi Rally at LA City Hall. For the most part the police stood back (including filming and photographing)from the counterprotesters. Early on (prob. around 1:10 or so) I saw one of the undercovers (the hispanic undercover, with bandana, in the first photo) holding a white male and walking him across the street to the L.A. Police Department where he was arrested. Also, at the same time another undercover (whose picture I didn't get) walked a black male across the street as well.
The second and third photos show a group of undercovers (somewhat led by the officer in the SF Giants hat and shirt). They witnessed numerous people throw stuff at the Nazis but didn't really do anything. I just saw the undercover in the SF hat take a photo of someone who threw something, but that was it.