A motorcyclist was caught on camera pinning down a police officer and repeatedly punching him, forcing passers-by to intervene.
The biker launched into the attack after being pulled over by a Merseyside Police traffic officer, the force said.
Dashcam footage recorded by a passing driver showed the two grappling in the road in the Litherland area of Liverpool on Thursday morning.
Three members of the public are seen in the footage struggling to break up the fight, before another police officer arrives and runs to help.
A 39-year-old man was arrested in connection with the violence.
A Merseyside Police spokeswoman said: “At around 11am an officer from the roads policing unit stopped a motorcyclist on Gorsey Lane, as the officer suspected the motorbike was being used without a valid MOT or road tax.
“The motorcyclist then assaulted the officer, before PCSOs and members of the public stepped in.
“The man was arrested on suspicion of assaulting a police officer and other motoring offences. He was taken to a police station and has been released pending further enquiries.
“The officer suffered cuts and bruises and remained on duty.”
It is one of several recent cases of Merseyside Police officers being assaulted, according to the force.
Acting Chief Inspector Keith Kellett said: “Thankfully the officer was able to remain on duty and suffered only minor injuries, and we’d like to thank the members of the public and other officers who came to his assistance.
“This officer was simply trying to do his job and I’m sure the public would agree that officers who are keeping our roads safe, should not be subject to behaviour like this.”