The fan at the centre of last weekend’s devastating Tour de France pile-up has spoken out about what she called an act of “stupidity”.

The spectator, a 30-year-old Frenchwoman, was arrested and taken into custody on Thursday by police in Brittany, four days after the accident brought down dozens of riders and forced German rider Jasha Sutterlin to abandon the race.

She is accused of involuntarily causing injury and putting lives at risk after brandishing a cardboard sign while leaning into the path of speeding riders. The woman appeared to be looking in the other direction, apparently at a camera, rather than at the approaching peloton.

She faces up to two years in jail, depending on the seriousness of the cyclists’ injuries.

German Tony Martin was one of those sent tumbling when he rode straight into the sign. A large number of other riders also fell.

The woman, whose name has not been released, faded into the crowd with her sign in the chaos after the crash. She had not been seen for days, until police tracked her down based on accounts from people they had questioned this week.

Local police chief Nicolas Duvinage said a social media appeal for witnesses led to more than 4000 messages, some of which “were verging on incitement to violence”, according to AFP.

The pile-on from cycling fans has reportedly left the woman feeling scared and worried about repercussions.

“The suspect has expressed her feelings of shame and fear about the consequences of her act,” local prosecutor Camille Miansoni said at a briefing in Brest that was broadcast live by some French television channels.

“She said she is worried about the media attention for what she has called her ‘stupidity’,” Mr Miansoni said.

He said the fan had intended only to send an “affectionate message” to her grandparents. Her sign read “Allez opi, omi” (Go grandpa, grandma).

Mr Miansoni said the woman had no criminal record and was scared and ashamed after what she said was an “idiocy”.

Police also called for the spectator not to be lynched on social networks.

“We are withdrawing our complaint. This story has been blown out of proportion but we wish to remind everyone of the safety rules on the race,” Tour director Christian Prudhomme told Reuters.

“If you come to the Tour, you hold your kid, you hold your pet and don’t cross the road carelessly. And above all, you respect the riders – they’re the ones worthy of live TV.”

Writing on Instagram after the crash, Martin criticised people “next to the road who think that the Tour de France is a circus”.

“Please respect the riders and the Tour de France!” he wrote.

“Use your head or stay home!”

Mr Miansoni said riders injured in the crash could still file lawsuits later.

On Tuesday, Tour de France riders brought the race to a halt for about a minute one kilometre into its fourth stage in a silent protest for safer racing conditions after numerous crashes.

-with AAP