A knife-wielding man sexually assaulted a girl in Overton Park on Wednesday.
The young man approached a pair of girls and inappropriately touched one of them at about 4pm.
He then approached a group of older boys before leaving the scene.
A passerby spotted the man with the knife and called police but he left the scene before officers arrived.
Anyone who uses the park has been advised to walk in groups.
A 14-year-old boy has been arrested on suspicion of assault in connection with the incident.
If you have information about the incident, you can call police on 101 or the charity Crimestoppers on 0800 555 111.
- Sutton Council appeals court ruling that it was responsible for Beddington Park bridge fall that left cyclist paralysed
- UPDATE: Man 'carrying gun' arrested by firearms police in Benhilton
- REVEALED: Arlo and Ellie are hot - but what are the other most popular baby names in 2016 so far?
- Liberal Democrats hold Carshalton Central in Sutton Council by-election
- 'Let her rest in peace': Granddad's anguish after killer dad appeals conviction for murder of six-year-old Ellie Butler