WHAT'S up dude? A little rioting action it seems.

At least eight arrests were made when violent behaviour broke out at Hunting Beach on Sunday when at the close of the US Open of Surfing.

Local TV station KTLA 5 reported that the violent behavior erupted around 7pm at Main Street and Pacific Coast Highway near the Huntington Beach Pier

A video posted to YouTube shows looters breaking into a bike shop, and turning over portable loos and road signs.

Some people were throwing bottles and others were fighting, and at least one store was looted.

Somebody stole a bike before others in the crowd persuaded the looters to stop, witnesses said.

It remained unclear exactly what prompted the violence to start.

"It really started when a ketchup bottle came from the top story of a bar and exploded," witness Kyle Calder said.

"Kid didn't know what to do with it, threw it into the crowd. Couple more fights broke out. That's when the cops came, and everything went mayhem from there."

The LA Times reported that police with riot helmets and nightsticks closed in on Main Street, video shows.

Some in the crowd coughed and wiped their eyes from the gas police fired to disperse the crowd. Some covered their mouths and noses with shirts.

The U.S. Open of Surfing draws thousands of people to Huntington Beach. 

BOM issues hazardous surf warning

Premium Content BOM issues hazardous surf warning

Surfers, fishers, swimmers and boaters should avoid the ocean.

Fireworks and 100 laps for Australia Day double-header

Premium Content Fireworks and 100 laps for Australia Day double-header

Motorsports fans will be cheering on future stars and established champions at the...

‘Suffering in silence’: Bluesfest boss’ cry for help

Premium Content ‘Suffering in silence’: Bluesfest boss’ cry for help

“It’s time to raise our voices. Let us not forget what we need to do.”