The spectator who caused a mass crash of riders on the opening day of the Tour de France has been arrested, a source close to the investigation said on Wednesday.

"The woman has been formally identified and she was taken into custody a few minutes ago," the source told AFP after the state prosecutor for the city of Brest in Brittany, Camille Miansoni, said "a suspect is in custody".

On Saturday, a woman brandishing a cardboard sign on the roadside between Brest and Landerneau caused German rider Tony Martin to fall, who in turn brought down dozens of riders behind him.

The crash held up the race for five minutes while bikes and bodies were untangled.

The incident and a series of crashes on the second and third stages of the race angered teams and led to the entire peloton staging a symbolic protest, briefly halting at a pre-arranged point, at the start of Tuesday's fourth stage.