The fan held a large placard out towards the TV cameras during the event, seemingly oblivious to the proximity of the oncoming riders.
Jumbo-Visma’s Tony Martin stuck the placard as he rode past and fell to the ground, causing a mass pile-up of bikes and riders in his wake as the entire peloton came to a halt.
According to L’Equipe, police are now looking for the spectator, who is reported to have fled the scene, as part of a criminal enquiry into the incident.
The police said they intend to charge the individual with “unintentional short-term injury through a manifestly deliberate breach of a duty of safety or care”.
The incident caused chaos during the opening stage of the competition on Saturday (June 26).
It took a full 25km for the bruised riders to fully regroup, only for the pack to be pulled apart again by a high-speed crash on the downhill approach to Landerneau in France.
British cyclist Chris Froome was among the worst affected and struggled to get back to his feet following the incident.
Froome did eventually make it to the finish, crossing the line 14-and-a-half minutes down, and was sent for scans after the race.
Writing about the incident on Twitter, the 36-year-old said it was not how he planned the race to go, but was pleased to be able to finish.
He added: “No fractures thankfully but there’s a lot of swelling & bruising to my left leg and thorax. I’m going to try get a good night’s rest & see how things look in the morning before making any decisions with the medical team. Thanks for the many messages of support X."
Meanwhile, three riders – Jasha Sutterlin, Cyril Lemoine and Ignatas Konovalovas – failed to finish and others faced an uncomfortable night’s sleep nursing injuries.
The overall hopes of several riders already appear over due to huge time losses, most notably Miguel Angel Lopez who is almost two minutes down.