The woman, whose name has not been made public, scrawled “AE911” in six centimetre-high letters in indelible felt-tip marker across Delacroix’s most famous work just
before the Louvre-Lens closed on Thursday, sources say.
Another visitor and security guard immediately grabbed her and she was still in police custody on Friday morning.
She has not made any statement on the reason for her act but AE911 is the name of a website carrying a petition calling for an independent inquiry into the 2001 attacks on the Pentagon and New York's World Trade Center.
A specialist restorer was rushed to the scene and the Louvre believes that the inscription can be removed.
Liberty the leading the people, a work that is nowadays photoshopped almost as often as it is reproduced, is possibly the best-known image of French popular resistance to tyranny.
Delacroix painted it in 1830, following three days of insurrection on 27, 28 and 29 July of that year.
It is one of the prestigious works, along with Raphael’s Portrait of Balthazar Castiglione and Georges de la Tour’s Magdalen with the smoking flame, that have been leant to Louvre-Lens for a year to help launch the museum’s regional offshoot.