A man has died after taking part in a 5km timed run.
The 43-year-old collapsed on Saturday morning during the Forest Rec Park Run in Forest Recreation Ground, Nottingham - a weekly event, which provides time trials for all participants in the circuit.
Fellow runner James Herbert, 33, said he was one of three people who performed CPR on the man, who had been running 30 metres in front of him when he fell to the ground.
He said: "We'd just got to the top of the hill and he, all of a sudden, fell flat on his face.
"Myself and two other runners - they were both doctors - ran over to him, flipped him over and started doing chest compressions."
Despite paramedics arriving to continue resuscitation efforts, the man was taken to Queen's Medical Centre where he was pronounced dead at around 10.30am.
Mr Herbert, a chemistry teacher who lives in Nottingham, said he had looked "the picture of health, apart from the fact he was in a really bad state".
Nottinghamshire Police said the death is not being treated as suspicious and a report will be submitted to the coroner.