A 16 year-old youth is in a critical condition after a collision between a marked police car and a moped being ridden by three male teenagers.
Another 16-year-old suffered a serious leg injury and a 15-year-old had minor injuries from the crash in the early hours of Sunday in South Park Road near the junction with Trinity Road in Wimbledon, south west London.
Scotland Yard said the moped was being tracked by the National Police Air Service helicopter at the time of the incident as it was believed to have been involved in an earlier attempted robbery. The moped had been reported lost or stolen to police on July 12.
All three teenagers were arrested at the scene where two large knives were recovered.
The teenagers were taken to a south London hospital for treatment.
Scotland Yard said the Directorate of Professionals Standards has been informed and the incident has been referred to the Independent Police Complaints Commission.