The woman at the centre of one of the nastiest crashes in recent memory in the Tour de France has spoken for the first time about an incident she herself describes as an act of "stupidity".

Police in Brittany arrested her on Thursday, four days after the accident brought down dozens of riders and forced German rider Jasha Sutterlin to abandon the race.

The spectator brandished a large cardboard sign while leaning into the path of oncoming riders. She appeared to be looking in the other direction, apparently at a camera, and not at the approaching peloton.

The arrested spectator, moments before rider Tony Marin collided with her. (Photo / Sky Sport)

The woman, not publicly identified, was arrested in the Finistere region by police who tracked her down based on accounts from people questioned this week.

Local police chief Nicolas Duvinage said that an appeal for witnesses posted on Facebook led to more than 4000 messages, some of which "were verging on incitement to violence", according to AFP.

The extreme reactions from cycling fans has reportedly left the arrested spectator in a vulnerable state and scared of repercussions, especially from the media.

"The suspect has expressed her feelings of shame and fear about the consequences of her act," local prosecutor Camille Miansoni told reporters at a press conference 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'," Miansoni added.

Miansoni said she wanted to send an "affectionate message" to her grandparents, adding that the suspect's grandmother has German origins.

The fact the spectator has now been arrested and her actions have been widely condemned has led to Tour de France organisers dropping their legal action against her.

The aftermath of the crash. (Photo / Sky Sport)

Organisers made the decision in a bid to defuse the situation after the careless spectator was placed in custody, race spokesperson Fabrice Tiano said on Thursday.

Despite the Tour's decision, local prosecutors are still set to decide whether or not to file charges.

Tour organisers announced after the crash on the stage from Brest to Landerneau that they would start legal proceedings against the fan, who disappeared from the crash scene. She leaned into the path of veteran rider Tony Martin, whose fall had a domino effect on the peloton.

German rider Tony Martin remained in the race but suffered some nasty-looking injuries. (Photo / AP)

Fans gathering on the sides of roads and in villages as riders pass by is part of the tradition and charm of the Tour. The woman's sign read "Allez Opi-Omi," a mix of French and German-language terms of endearment for grandparents — "Go Grandpa-Grandma."

Meanwhile, Duvinage urged the public to calm down on social networks after several messages calling for violence were posted on the local gendarmerie Facebook page.

"The person being accused also has personal weaknesses and it is therefore advisable not to proceed to a media lynching or on social networks," he said.

- with AP