French police launched a call for witnesses Sunday into the giant crash that marred the opening stage at the Tour de France, caused by a spectator who leant into the path of the speeding peloton holding up a sign.

The woman held up a sign with 'Granny and Grandad' written in German and looked up the road towards the race's motorbike cameras, crucially, with her back to the speeding peloton.

German rider Tony Martin brushed into her, lost his balance and fell, causing a sickening domino effect that swept through most of the peloton, leaving them in a tangled mess of bikes and bodies.

"The major issue on the Tour is road security," lieutenant-colonel Joel Scherer of the French Gendarmerie told AFP on Sunday.

Another policeman told AFP that "zero fatalities" was their bottom line assignment aim on a Tour de France or any bike race.

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

DSM's German rider Jasha Sutterlin was the only cyclist who had to pull out at once, his Tour de France dream ruined on day one.

'Bad behaviour'

Eight other riders needed treatment from the official doctor and a host of others were treated for grazes, bruises and cuts caused by the pile-up.

Italian champion Sonny Colbrelli and Dutch rider Wout van Aert, who ran over Martin before falling head over heels, had both been among the favourites to win the first stage.

"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 do not spoil the show for everyone," he said.

The police said that 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 Scherer said.

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

The Tour started on Saturday from the Atlantic port of Brest.

Following the relaxation of French coronavirus restrictions the public turning out in massive numbers along the narrow Brittany roads for a stage eventually won by Frenchman Julian Alaphilippe.

"We witnessed inadmissable behaviour today with people running across the road in front of the race and children left to their own devices," race director Christian Prudhomme told French media. "You come here to see heroes. If you want to see yourself look in the mirror."

On Sunday, four official race vehicles preceded the peloton, which set off minus three more riders who were hurt in a separate crash later on Saturday, warning over-excited roadside fans to keep their distance.

"We are telling people to keep off the route and to control their children," Scherer said.