image copyrightGendarmerie du finistereimage captionThe woman's cardboard sign caused one of the worst crashes in Tour de France history

French police say they have arrested a woman suspected of causing a huge crash during the Tour de France by waving a sign in the riders' path on Saturday.

The peloton was 45km (28 miles) from the end of the first stage, when her sign clipped German rider Tony Martin.

He fell to the ground and caused dozens of other riders to follow suit, in what was one of the Tour de France's worst ever crashes.

French authorities say the woman, who is French, is in custody in Landerneau.

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Video footage of the incident has been shared widely online.

The woman can be seen holding a sign with "granny and granddad" written in German. She is looking away from the peloton coming towards her and does not see them approach, while holding her sign too far into the road.

As a result of the crash, one rider had to pull out of the Tour completely and another eight riders were treated for injuries.

The worst Tour de France crash I've ever seen pic.twitter.com/1jngQE1pYg

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The crash held up the race, which was between between Brest and Landerneau in north-west France, for five minutes, while bikes and riders were untangled and cleared from the road.

Following the incident, Tour de France Deputy Director Pierre-Yves Thouault said the tour would take legal action against the woman.

"We are suing this woman who behaved so badly. We are doing this so that the tiny minority of people who do this do not spoil the show for everyone," he told AFP news agency.

France has relaxed its coronavirus restrictions and many people have flocked to see the riders.

Those hoping to watch the race have been told to keep off the route and maintain their distance.