Claims a 15-year-old boy had his arm broken by a police officer outside a supermarket, are being investigated by a watchdog.
The teenager was allegedly "taken to the ground" outside Morrisons in Shefford, Bedfordshire, on 14 December.
The Independent Office for Police Conduct (IOPC) said it would examine the level of force used and the actions of other officers at the scene.
It will also assess the care given to the boy after he was injured, it said.
Several officers and teenagers were involved in the incident outside the St Francis Way supermarket at 18:00 GMT.
The injured boy was taken home and no further police action was taken.
Bedfordshire Police referred the matter to the IOPC.
IOPC regional director Sarah Green said: "A teenage boy has sustained a serious injury while being detained by police and it is vital we establish what happened and how officers reacted afterwards.
"This would have been a busy time at the supermarket in the run-up to Christmas and I would urge anyone who may have seen this incident to contact the IOPC as soon as possible to assist in our investigation."