The Oakland police department and city websites were offline on Wednesday and activist hackers from the Anonymous collective who threatened to take action against law enforcement over the Bay Area demonstrations claimed responsibility for the outage.

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, Reuters reports.