Officers were called to Adventure Island at around 3pm on Thursday (1 June) following reports of an 11-year-old girl being sexually assaulted whilst on a ride at the amusement park.
The suspect was described as a white man, in his late 50s, with short, grey hair and wearing a green t-shirt and sunglasses.
The victim is receiving support from specialist officers.
Investigating officers from Southend Criminal Investigation Department have arrested a 57-year-old man from Chelmsford in connection with this investigation. He has since been released on bail whilst enquiries continue.
We are appealing to anyone who may have witnessed this assault or have any further information to contact us.
You can let us know by submitting a report on our website or by using our online Live Chat service available Monday to Friday (excluding public holidays) between 10am-9pm.