California police on Monday were investigating a violent clash between members of a white supremacist group and counter-protesters on Sunday during which 10 people were stabbed.
The brawl in the state capital, Sacramento, fuels concerns about the risk of violence outside the Republican and Democratic party conventions in Cleveland and Philadelphia next month, and in the run-up to the Nov. 8 presidential election.
The Traditionalist Worker Party, a white nationalist group, had called the rally in part to protest against violence that has flared outside recent rallies by presumptive Republican presidential nominee Donald Trump, one of its leaders told Reuters.
Sunday's fight broke out at about noon at the start of the rally outside the California State Capitol, the Sacramento Police Department said in a statement.
Self-styled militants protesting the white nationalist rally confronted a local news reporter, shoving him, knocking his microphone to the ground and taking his phone and notepad, police said.
"Protesters swarmed the Capitol from the west side, then north and south," the reporter, Mike Luery of NBC affiliate KCRA, said in a Twitter post on Monday. "We were attacked on the north side."
More than 100 Sacramento police officers were deployed at the event after authorities learned that a group called Anti-Fascist Action Sacramento was planning a counter-protest, police said.
Protesters tried to block Traditionalist Worker Party members from reaching a staging area where they had been given permission by the California Highway Patrol to demonstrate between noon and 2 p.m. local time, police said. Violence quickly ensued, police said.
The Sacramento Fire Department said 10 patients were treated at area hospitals for multiple stabbing and laceration wounds.
At least two assaults took place in the city's jurisdiction and are being investigated by city police. The rest of the attacks are believed to have happened on Capitol grounds and will be investigated by California Highway Patrol, police said.
No arrests have been made.
Since the clash, both groups have said their supporters were wounded.
"Regretfully, one of our men was injured and is currently in the hospital," the Traditionalist Worker Party said in a statement on its website.
Anti-Fascist Action Sacramento said it had suffered a greater number of wounded.
"So far, we know of six comrades who have been injured to a degree that required hospitalization," the group said on its Facebook page.
Video footage on social media showed dozens of people, some wearing masks and wielding what appeared to be wooden bats, racing across the Capitol grounds and attacking others.