Officers were conducting a foot patrol near Beach Street and Royal Road in the area of Sheppey Leisure Centre in Sheerness, Kent, when they began approaching the 18-year-old moped rider.
Kent Police said that as officers walked towards the rider, the vehicle left via an alleyway adjacent to the leisure centre and was subsequently involved in a collision.
First aid was provided for the man, from Sheppey, but he was pronounced dead at the scene.
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The boy, 16, who was travelling as a passenger, was taken to a London hospital where he remains in a critical condition.
A mandatory referral was made to the Independent Office for Police Conduct and an investigation is under way.
