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Ten people have been injured, two of them critically, in chaotic clashes between two rallies in Sacramento, California.
A march by a far-right group descended into chaos when the demonstrators were attacked by counter-protesters.
Local media reported several people with stab wounds.
Traditionalist Worker Party members, described as a neo-Nazi group, rallied outside Sacramento's Capitol building when anti-fascist group Antifa confronted them.
Sacramento authorities called the situation a "mass casualty incident".
The Sacramento Fire Department said on Twitter that five people were hospitalised, "some with critical trauma stab wounds". There are no reports of arrests made.
When the white supremacists arrived at the capitol building at about noon on Sunday, "counter-protesters immediately ran in - hundreds of people - and they engaged in a fight," California Highway patrolman George Granada told Reuters.
The incident may fuel worries about the possibility for more violence at presidential campaign meetings leading up to the November 8 election.
Traditionalist Worker Party chairman Matthew Heimbach was seen on camera threatening a black woman at a Donald Trump rally earlier in 2016.