At least 10 people were injured — two of them seriously — at a rally outside the California state capitol in Sacramento on Sunday as members of a white supremacist group clashed with counter-protesters, authorities said.

The melee erupted during a rally staged by the Traditionalist Worker Party, described by the Southern Poverty Law Center as a white nationalist extremist group.

One of its leaders, Matt Parrott, said the party had called the demonstration in part to protest against violence that has broken out outside recent rallies by Donald Trump, the presumptive Republican presidential nominee.

The incident may fuel concerns about the potential for violent protests outside the major party conventions in Cleveland and Philadelphia this summer and in the run-up to the Nov. 8 presidential election.

When about 30 white supremacists arrived at the capitol building around noon on Sunday, "counter-protesters immediately ran in — hundreds of people — and they engaged in a fight," said George Granada, a spokesperson for the Capitol Protection Service division of the California Highway Patrol.

Granada estimated there were about 400 counter-protesters involved. As people tried to leave the area, smaller fights broke out, he added.

Richard Becker protests after multiple people were stabbed in Sacramento, Calif., during a clash between neo-Nazis holding a permitted rally and counter-protesters on Sunday. (Max Whittaker/Reuters)

In announcing the counter-protest, a group called Anti-Fascist Action Sacramento said on its website that it had a "moral duty" to deny a platform for "Nazis from all over the West Coast" to voice their views.

"We have a right to self-defence. That is why we have to shut them down," Yvette Felarca, a counter-protester wearing a white bandage on her head, told reporters after the clash.

Sacramento Fire Department spokesman Chris Harvey said nine men and one woman, ranging from 19 to 58 years old, were treated for stab wounds, cuts, scrapes and bruises. Of the injured, two were taken to the hospital with life-threatening stab wounds.

"There were a large number of people carrying sticks and rushing to either get into the melee or see what was going on," he said.

There were no immediate reports of arrests and the building was placed on lockdown until protesters cleared the area.

Final from State Capitol incident: 10 patients, 9 M, 1 F, 19 to 58 age range. Multiple stab and laceration wounds. <a href="https://t.co/r3aSvrGH4H">pic.twitter.com/r3aSvrGH4H</a> —@SacFirePIO

'We are white nationalists'

Matthew Heimbach, chairman of Traditionalist Worker Party, said his group had expected violence even though it planned a peaceful rally and had a permit.

"We were there to support nationalism. We are white nationalists," Heimbach told Reuters. "We were there to take a stand."

Video footage on social media showed dozens of people, some of them wearing masks and wielding what appeared to be wooden bats, racing across the capitol grounds and attacking others.

Photos on social media showed emergency officials treating a victim on the grass in the area as police officers stood guard.

Officials about 30 members of the Traditionalist Worker Party gathered at the capitol for a rally Sunday when they were met by about 400 counter-protesters. (Max Whittaker/Reuters)

The melee comes about four months after four people were stabbed during a scuffle between members of the Ku Klux Klan and counter-protesters near a KKK rally in Anaheim, Calif.

In recent months Trump has blamed "professional agitators" and "thugs" for violence that has broken out at many of the Republican candidate's rallies.

In Albuquerque, New Mexico, last month, anti-Trump protesters threw rocks and bottles at police officers who responded with pepper spray. A month earlier, some 20 demonstrators were arrested outside a Trump rally in Costa Mesa, Calif.