A six-year-old boy was indecently assaulted in public toilets at Colchester in Essex on Saturday.
A police spokesman the boy was very shocked at the assault in the toilets of the town's Castle Park at about 1600 BST on Saturday afternoon.
Det Sgt Roger Napier, from Essex Police, said it was a traumatic experience for the child.
His attacker is described as tall, white, thin, in his late 20s, and with gingery/black hair.
Police are urging anyone who may have any information about the assault to contact them.