Oakland and neighboring Berkeley have been the site of nightly protests against decisions by two grand juries not to charge white police officers in the killings of unarmed black men in Ferguson, Missouri, and New York City.
Police in riot gear faced off with hundreds of demonstrators and made scores of arrests. At times they have fired tear gas and pepper spray against protesters, some of whom threw stones at the officers and damaged property.
The police response prompted Anonymous, an amorphous group that conducts multiple hacking campaigns at once, to issue a video warning to the Berkeley Police Department on Tuesday.
In the three-minute recording, a masked and hooded spokesman reads a script in a robotic-sounding voice, accusing officers of intimidating and attacking harmless, peaceful protesters.
"Innocent citizens have been subjected to multiple counts of brutality ... We call on the Berkeley P.D. to cease and desist," the figure says. "We will no longer tolerate abuses of our constitutional rights, nor will we stand idly by observing protesters being assaulted."
The video shows grainy footage of what the narrator says is a Berkeley police officer using his baton to strike a protester who has his hands raised during a demonstration on Saturday.
The video names the policeman, and gives his badge number.
"If you do not bring this officer to justice, we will take action," the spokesman for the hackers says.
On Wednesday, the Berkeley Police Department's website was functioning, but the sites for the Oakland Police Department and
City of Oakland were both down.
A Twitter account associated with Anonymous took the credit, posting the message: "Tango Down: oaklandpolice.com" while also using the protesters' hashtags #ShutItDown and #ICantBreathe.
Officials with the city of Oakland and its police department could not immediately be reached for comment.