Emergency crews transported six people who were stabbed outside the California state Capitol building in Sacramento, the city's Fire Department said.
Spokesman Chris Harvey said a rally by Ku Klux Klan and other right-wing extremists groups turned violent on Sunday when they were met by counterprotesters.
A large fight broke out after people carrying sticks rushed into the area where the rally was being held.
Mr Harvey said California Highway Patrol officers managed to break up the large fight but that some members of both groups remain in the area.
#BREAKING Sac Fire just confirmed multiple people have been stabbed at the Capitol protest. @ABC10 pic.twitter.com/afuc69Sw5r
— Frances Wang (@FrancesWangTV) June 26, 2016