Image copyright Weymouth police Image caption The incident happened at the Bridport circuit
The police are investigating an incident involving a motorbike rider on track at the Weymouth Karting World Cup.
Officers said “aggressive” comments were made by a male rider on the track on Monday.
The rider approached riders at intervals for 10 minutes before a spectator pulled out a club and struck the rider, they said.
A post on the Weymouth police Facebook page said the rider “engaged” with the spectator, “who stood up to the rider”.
The rider is thought to have been suffering from “the condition of being punched in the face”.
He received medical attention.
The rider was removed from the track and taken to Weymouth hospital as a precaution, and the police investigation continues.
There have been fewer spectators at this year’s event at Bridport, Dorset, although races continued on Tuesday night.
Avon and Somerset Police and Dorset Police did not return calls asking for comment.
Thousands of fans are expected at Weymouth’s royal track this weekend to watch the Sk8-Tough Kart Series taking place.
Last week, two women were injured when a car careered into spectators at the Bury St Edmunds 100, an annual track-side festival.