A 15-year-old boy was airlifted to hospital after suffering a head injury at Appleby Horse Fair.
Emergency services were called at around 6.40pm and told a teenager had jumped off the back of a moving vehicle and hit his head off the ground.
Police officers first on the scene provided first aid prior to handing over to paramedics.
The incident has occurred on Long Marton Road, close to the junction with Belgravia in Appleby.
An investigation is ongoing, police said.
The family of the teenager – who was taken from the scene to hospital by air ambulance – have been contacted and are aware of the incident.
A road closure is in place and police are asking for people avoid the area and thank the public for their patience.
Cumbria Police would like to hear from anyone who witnessed what occurred and has yet to come forward.
Report online at www.cumbria.police.uk/reportit or call 101.
