A 23-year-old man has died after collapsing during the Brighton marathon.
Medics and an off-duty police officer tried to revive the runner, from London, before he was taken to hospital, but he was later declared dead.
He is thought to have had a cardiac arrest around the 16-mile mark of the course.
A Sussex Police spokesman said: "A 23-year-old man has sadly died after suffering a medical episode whilst competing in the Brighton marathon this morning.
"Emergency services were called to Grand Avenue in Hove at around 11.45am where it is believed the London man suffered a cardiac arrest.
"He was treated at the scene by medical staff, including doctors and paramedics, along with an off-duty police officer, before being taken by ambulance to hospital, where he underwent surgery but sadly he couldn't be saved and died a short time later."
The man's family has been informed of his death.
A spokesman for organisers of the marathon said: "It is with regret that we can confirm that a competitor collapsed and later died competing in the Brighton Marathon this morning.
"Our emergency plans were put in place and he was treated at the scene by senior doctors, nurses and paramedics before being taken by ambulance to hospital.
"Despite receiving immediate attention, the runner was later declared dead at the Royal Sussex County Hospital."
He added: "We at Grounded Events, the organisers of the Brighton Marathon, would like to express our sincere condolences to family and friends of the deceased."
A spokesman for the Royal Sussex County Hospital said the man died at 2.30pm.
England cricketer Matt Prior had officially started today's race, the fourth Brighton marathon, in Preston Park at 9am.
Organisers said 9,315 competitors were taking part.