@alfiekohn
Get the detainee's name, birth date, and country of origin, record as many details as possible (license plates, location, time, number of "ICE agents" & any identifying info...). Call an immigration defense hotline ASAP and tell them everything. Exchange contact info with any other witnesses.
Assume the police will not help and/or ICE will be gone by the time they arrive and/or they will show up and essentially run security for the raid. So far there are literally no instances of police intervening to stop an "ICE" abduction that I'm aware of (please correct me if wrong).
In Los Angeles:
ImmDef Immigrant Defenders Law Center : (213) 833-8283
Los Angeles Rapid Response Network: (888) 624-4752