From the The Telegraph:
The rule banning women from dressing like men – namely by wearing trousers - was first introduced in 1800 by Paris' police chief and has survived repeated attempts to repeal it.
The 1800 rule stipulated than any Parisienne wishing to dress like a man "must present herself to Paris' main police station to obtain authorisation".
In 1892 it was slightly relaxed thanks to an amendment which said trousers were permitted "as long as the woman is holding the reins of a horse".
Then in 1909, the decree was further watered down when an extra clause was added to allow women in trousers on condition they were "on a bicycle or holding it by the handlebars".
In 1969, amid a global movement towards gender equality, the Paris council asked the city's police chief to bin the decree. His response was: "It is unwise to change texts which foreseen or unforeseen variations in fashion can return to the fore."
As Evelyne Pisier, a law professor whose book Le Droit des Femmes (The Rights of Women) unearthed the curious decree points out, given that trousers are compulsory for Parisian policewoman, they are all breaking the law.