The Mont Blanc Ultra Trail, a day and night race around the highest summit in Europe, attracted around 7,500 runners as it began on Friday.
Billed as the race every runner should complete at least once in their life, several runners said they were apprehensive about finishing the race.
One runner said: “I’m scared to death! Every time I ask myself: ‘Why am I going on a such a thing?’ 168 kilometres really is for mad people, but I’ve trained for this, so I hope it will be ok! Last time it didn’t work.”
Starting in the centre of Chamonix on Friday evening, some will complete the 168 kilometre trail in as little as 12 hours, although most take much longer.
The race sees participants climb as high a 9,600 metres and takes in the incredible scenery of the French, Swiss and Italian sides of the Alps along the way.