A spectator suffered serious head injuries when he was hit by a car taking part in a rally.
The driver of the car also suffered injuries in the collision at Dixies Corner, near Sennybridge, mid-Wales, and both men were taken to hospital.
A spokeswoman for the Welsh Ambulance Service said: "The spectator was transported to the University Hospital of Wales, Cardiff, via air-ambulance.
"The other patient was taken to Prince Charles Hospital, in Merthyr, with minor injuries."
Inspector Adrian Evans, of Dyfed-Powys Police, confirmed they were investigating an incident at the rally in conjunction with South Wales Police.
Inspector Evans said the injuries suffered by the spectator, a man in his 60s, were believed to be life-threatening.
He said: "Dyfed-Powys Police are appealing for any witnesses to the incident to contact them.
"The police are particularly keen to have contact with any person that may have any images made at the time, immediately prior to or after the incident."
Anyone with information can contact the force on 0845 3302000 or via any police station.
The Harry Flatters Rally was taking place in the area at the time of the incident.
