Jamilah Nasheed, a Missouri state senator, was arrested this week while outside the Ferguson police station protesting the August shooting death of an unarmed black teenager.
Nasheed was taken into custody by Ferguson police officers Monday, said St. Louis County Police spokesman Brian Schellman.
Further details were not immediately available, Reuters reported.
CNN footage broadcast by television station KSDK showed the politician shouting "No Justice," with replies of "No peace," from several protesters, as she was handcuffed along with another man.
KDSK reported that Nasheed was arrested after standing in the street and blocking traffic, then failing to move despite repeated warnings from police.
Demonstrations have gripped the St. Louis area almost daily since the Aug. 9 killing of unarmed black teenager, 18-year-old Michael Brown, by white police officer Darren Wilson.
A grand jury is considering charges against Wilson, who has not spoken publicly about the matter. Protesters want Wilson arrested immediately and have called for the appointment of a special prosecutor.