A “young man” was reportedly assaulted in Grantham Market Place amid crowds of people attending the fair.
Grantham Police have appealed for information about the incident which is said to have happened on Monday March 10 between 6.45pm and 7pm.
A police CID spokesman said: “The town was very busy. There would have been lots of people who witnessed the incident.
“Therefore we are looking for witnesses to the assault.”
However, for legal reasons, the police say they cannot give further information, other than the victim was a young man or teenager.
Witnesses to the alleged assault are asked to ring 101 and quote incident 343 of March 20 2018.