Oregon Governor Kate Brown has accused US federal agents in unmarked cars who apparently detained protesters in Portland of a “blatant abuse of power”.
Federal officers, deployed by President Donald Trump, have also fired tear gas and less-lethal munitions into crowds of demonstrators.
Homeland Security Secretary Chad Wolf called the protesters a “violent mob”.
Activists have been protesting against police brutality since George Floyd’s killing in police custody on 25 May.
On Friday evening local time, Oregon Attorney General Ellen Rosenblum said that the state justice department was filing a lawsuit against the federal government over the detention of protesters “without probable cause”.
“These tactics must stop,” Ms Rosenblum said in a statement. “They not only make it impossible for people to assert their First Amendment rights to protest peacefully, they also create a more volatile situation on our streets.”