Sheriff's Sgt. Ray Kelly says about 300 people staged a peaceful protest Friday at the Alameda County Fairgrounds in Pleasanton, where the Urban Shield expo runs through Monday.
Some people chained themselves together to block the main gates to protest what activists call the militarization of police.
Kelly says about 20 people were arrested later for blocking a roadway and failing to disperse. Organizers put the count at 23. The arrestees were cited and released.
Billed as the nation's largest tactical exercise, Urban Shield offers dozens of training sessions for police, firefighters and other first responders, with scenarios ranging from active shooters to ocean rescues.