The incident happened at Rawson’s Rake in Ramsbottom as riders took part in the Rake Hill Climb on Sunday.
Several bystanders had come to the aid of the man who had collapsed during the event, until a crew from Lancashire ambulance service attended.
The man was surrounded by quick-thinking spectators who had attempted to conduct CPR. Cyclists also stopped due to their concern for the welfare of the individual.
A spokesperson for Lancashire Road Club, which organises the event, said: “During today’s event, a spectator became unwell. The event was immediately suspended while medical assistance was provided. Emergency services attended promptly, and the individual was taken to hospital.
“As a mark of respect, the event was cancelled. Lancashire Road Club would like to thank the medical professionals and emergency services involved, and our thoughts are with the individual and their family.”
Dozens of cyclists had flocked to Ramsbottom from around the region to take part in the annual event on Rawson’s Rake.
The route length sits at just under 1km, taking place between Ramsbottom and Holcombe village.
It is known in British cycling as one of the most challenging short hill climbs in the country.