A teenager was stabbed in an incident at Teddington Lock on Friday evening.
The 14-year-old male suffered injuries, believed not to be life-threatening, during a suspected robbery.
Large crowds of youths were dispersed and a knife was recovered from the scene.
A Section 60 order was put in place on Friday evening, providing officers with additional search powers.
London Ambulance Service paramedics also attended, but were unable to locate the victim of the stabbing.
Police were later made aware of a young male attending hospital with a small stab wound at around 9pm.
Police have arrested two 15-year-old males on suspicion of GBH and robbery.
Both have been released on bail and will return to the police station in early June.
The incident is believed to be an isolated one, involving a small group of males.
Enquiries are ongoing and anyone with information is asked to call police on 101 or tweet @MetCC, providing the reference CAD7586/14MAY.