At least four people have been stabbed in a brawl between Ku Klux Klan (KKK) members and counter-protesters in Anaheim, California, US police said.

Thirteen people, both KKK members and rival demonstrators, were arrested

The violence broke out when several Klan members arrived at a local park for a rally and were attacked by counter-protesters, according to police.

One of the stabbing victims is said to be in a serious condition.

A Klansman who allegedly stabbed one of the demonstrators with a flag pole is among those detained.

The counter-protests had been underway for hours before the KKK members arrived, witnesses reported.

As the Klansmen unpacked signs they were said to have been surrounded by demonstrators.

Some were seen kicking a man whose shirt read "Grand Dragon", the name of a high-ranking KKK member, reports said.

'Not necessary'

"A lot of us were trying to break it up," one witness, Dion Garcia, told Reuters.

"This was not necessary, they should've just let the Klan protest. This is America, we have free speech."

Another witness, Brian Levin, said after helping a Klansman away from the crowd, he asked him "how does it feel that your life was just saved by a Jewish man?"

"Thank you," the man replied, according to Levin.

The Ku Klux Klan is one of the oldest and most infamous hate groups in the US, and has targeted African Americans, Jews, Catholics and immigrants, according to the Southern Poverty Law Center, which tracks hate groups.

The group has a long history in Anaheim, with KKK members elected to the city council in the 1920s.