Three people were stabbed, one of them critically, and 13 others were arrested in California on Saturday when a Ku Klux Klan rally erupted into clashes with counter-protesters, witnesses and police said.

Around midday, half a dozen members of the white hate group arrived at the protest site in Anaheim where they were "swarmed" by protesters, who attacked them with at least one wooden plank, witness Brian Levin told AFP.

"Preliminarily, it appears six KKK people arrived and were immediately attacked by counter-protesters, which led to a counter-protester being stabbed," local police spokesman Sergeant Daron Wyatt said.

The initial clash spawned several separate fights, Wyatt added, noting that the three stabbing victims were all counter-protesters, while another two KKK members were stomped by the crowd.

Of those arrested, six were KKK members and seven were from the rival demonstration, Wyatt said. All were men, except for one woman in each group.

The counter-protesters "smashed the side window of the Klan SUV and the front windshield," said Levin, who heads the Center for the Study of Hate and Extremism at California State University in San Bernardino.

"At that point, the crowd got extremely violent."

Some of the protesters started kicking a man wearing a shirt that read "Grand Dragon," said Levin, describing the KKK members' wounds as ranging from minor to significant.

"He was kicked when he was down" on the ground, he said.

Levin said he had attended the rally as an observer, but ended up standing in between the Klansmen and the crowd of angry protesters to try to stem the violence.

"I helped two Klansmen get out of the way," said Levin as he headed to the police station to give his witness statement.

"I told the crowd: Dr (Martin Luther) King wouldn't approve of this, please don't harm these men."

After pushing a Klansman away from the crowd, Levin said he asked him: "How does it feel that your life was just saved by a Jewish man?"

The man replied "thank you," according to Levin.

There are currently between 5,000 and 8,000 KKK members, according to the Southern Poverty Law Center.

Although it has historically targeted blacks, the Klan has also launched attacks against Jews, immigrants, gays and lesbians. Until recently, it attacked Catholics as well.

It was formed in 1865.