Delacroix Liberty painting defaced in Louvre
She was held after being seen scrawling a graffiti tag on the painting, a Romantic masterpiece painted in 1830 to celebrate a French uprising.
The museum in the northern town of Lens said the work might easily be cleaned but would be examined by a restorer.
The Louvre Lens museum only opened in the former mining town in December.
The painting by Eugene Delacroix, which featured on the pre-euro, 100-franc French banknote and reportedly inspired the Statue of Liberty in New York, is being exhibited in Lens for a year.
French media quoted unnamed legal sources as saying the graffito was a clear reference to a 9/11 conspiracy theory.
The gallery remained closed to the public on Friday.
'Unstable'
Just before closing time the previous day, a 28-year-old woman scrawled the 30cm (12in) graffito on the bottom of the painting and was immediately detained by a museum guard, France's 20 Minutes news website reported.
The work itself, which commemorates the July Revolution of 1830, measures 325cm by 260cm.
The mark may be "easily cleaned" but a restoration expert was being sent from the parent museum in Paris, the museum said in a statement.
No decision has yet been taken on whether the painting will have to be removed, the museum was quoted as saying by French broadcaster France 3.
The local prosecutor, Philippe Peyroux, told AFP news agency that the woman in custody appeared to be "unstable" and that he had requested a psychiatric examination.
He added that the woman, whose identity has not been released, had a "French-sounding name".
Source: BBC
