The incident took place at Morden Hall Park on Monday (April 14).
The victim-supervivor is being supported by specialized officers.
A 16 -year -old was arrested under suspicion of severe sexual assault and currently remains in police custody.
A police investigation is currently being carried out.
The police are appealing that any person with information is presented, and urges anyone who was near the Park near Morden Road in front of the medical center around 6 pm on April 14 to contact them.
The information can be informed by contact 101, citing reference 01/7379934/25.
The information can also be reported anonymously by contacting Crimetoppers at 0800 555 111.
