US cop punches, knees alleged road-rage driver in dashcam arrest

A US police officer is under fire after newly released dashcam shows him kneeing, punching and elbowing a suspect who was not resisting in any way.

Video released by the American Civil Liberties Union (ACLU) shows a plain-clothed policeman marching over to the car of a man suspected of initiating a road-rage incident in Worthington, Minnesota in July last year.

"Get the f--- out of the car, motherf---er. Show me your f---ing hands," shouts officer Joe Joswiak as he races towards to the 4WD with his gun drawn.

Joswiak then throws open the car door and starts punching Anthony Promvongsa, 22, as he sits in the car, still wearing his seatbelt.

The video shows Joswiak kneeing Promvongsa three times, as well as punching him and elbowing him.

As Promvongsa lies on the road being handcuffed, Joswiak pins his head to the asphalt with his knee.

"I did not even have the opportunity to take off my seat belt before I was literally blindsided with this unnecessary attack," Promvongsa said.

"I immediately pulled over for the Worthington squad car, and before I know what was happening, I was beat and ripped from my vehicle."

Joswiak has not faced any discipline what what the Minnesota ACLU described as "abhorrent treatment".

"Agent Joswiak’s use of force against Anthony Promvongsa is disturbing and completely unnecessary," Minnesota ACLU executive director Teresa Nelson said.

"We are calling for an investigation of Agent Joswiak’s behaviour and for him to be held accountable for his brutal attack ... up to and including termination and prosecution."

Promvongsa is accused of aggressive driving, tailgating an off-duty police officer's car, swerving and making hand gestures.

The Star-Tribune reports Promvongsa allegedly swerved towards Joswiak's unmarked police car.

But the dashcam video show Promvongsa immediately pulling over when he saw a police cruiser.

He has been charged with assault with a dangerous weapon, fleeing police in a vehicle, marijuana possession and driving after his license was revoked.

Promvongsa is now awaiting trial for his charges.

Joswiak has not been disciplined for his rough treatment.

Reckless police force is a particularly hot-button topic in Minnesota after a court acquitted an officer in Minneapolis for shooting dead Philando Castile late last week.