A surgeon working at the Tour de France has described the scenes after a horrific crash during the first stage as resembling a "war zone".

Gilbert Versier, a three-star general who has previously operated in Iraq and Afghanistan, has been an orthopaedic surgeon at the world's most prestigious cycling event for 11 years and was shocked by what he saw on the opening day.

A clueless spectator caused carnage when she stepped onto the road, oblivious to the oncoming peloton. Holding a sign and wanting to get in front of motorbike TV cameras, the woman had no idea a horde of riders was coming her way.

German rider Tony Martin brushed past her and was knocked off balance racing near the head of the peloton, and when he fell it caused a horrifying domino effect ending in a tangle of bikes and bodies.

At least 26 riders were injured on day one of the Tour.

What some are calling the "worst Tour de France crash ever seen" was described in troubling detail by Versier.

"It looks like a war zone, the same chaos, the same moans, bodies everywhere and tangled machines," Versier told French publication L'Equipe.

"You can't imagine so much damage. In the midst of the commotion, the riders getting up and wanting to start again, the most serious cases must be identified.

"In general, these are the ones who are furthest from the accident site, because they have been thrown."

French police have appealed for witnesses to come forward as they launched a probe into the giant domino-effect pile-up that marred the opening stage.

The woman who caused the crash reportedly fled the scene and event organisers are looking to sue her.

"We are suing this woman who behaved so badly," Tour deputy director Pierre-Yves Thouault told AFP. "We are doing this so that the tiny minority of people who do this don't spoil the show for everyone."

The woman, who was wearing a yellow raincoat, reeled away in horror when she realised the extent of her stuff-up, but she then disappeared into the deep roadside crowds, her sign folded away beneath her arm.

The police said when they catch the culprit they intend to charge her with "unintentional short-term injury through a manifestly deliberate breach of a duty of safety or care", Lieutenant-Colonel Joel Scherer of the French Gendarmerie said.

"We can confirm that no member of the public was injured and we have liaised with the race organisers on this," he said.

Meanwhile, Aussie star Caleb Ewan was involved in a nasty crash during stage three as several other riders also suffered injuries.

Geraint Thomas, the 2018 champion, lost his concentration and hit the ground so hard he dislocated a shoulder before making it back to the peloton with the help of three teammates.

Slovenia's Primoz Roglic then hit the tarmac hip first with 10km to go and while shaken he also limited his losses with the help of teammates. Although his Tour is not finished, he now has time to make up on Tadej Pogacar and Thomas.

The worst fall came in the home straight with Caleb Ewan hitting Tim Merlier's back wheel at over 80km/h and taking Slovak sprint specialist Sagan down with him, the pair sliding for tens of metres on the tarmac.