The graffiti tag made was later removed from the canvas without damaging the 1830 painting Liberty Leading the People

A woman has been arrested after defacing a painting by Eugène Delacroix at the Louvre satellite museum in Lens. The 28-year-old told police she had scrawled "AE911'' with an indelible marker on the painting, Liberty Leading the People, to draw attention to an organisation that appears to believe the 9/11 attacks were a conspiracy.

On Friday, after an art expert was dispatched from Paris to examine the painting, completed in 1830 to celebrate the July 1830 revolution, a museum official said the work had been cleaned and had suffered no lasting damage.

The 30cm graffiti tag had been removed from the lower right of the canvas without damaging the work, the official said. "The painting remains intact. The inscription was superficial; it was on the surface of the varnish and hadn't reached the paint layer." The cleaning operation had lasted two hours and had been done with the painting still on the wall. The wing of the gallery, closed for the emergency restoration, was expected to reopen on Saturday .

Delacroix's work shows a bare-breasted woman personifying Liberty brandishing a French tricolour in one hand and a bayonetted musket in the other, leading the people forward over the bodies of the fallen.

Philippe Peyroux, the local prosecutor, told AFP that the woman held by police appeared to be "unstable". He said he had requested that she be examined by a psychiatrist. He added that she had a "French-sounding name", but her reasons for vandalising the painting were not clear.

"Is this a person who acted under the influence of some kind of frenzy or is it some kind of demand? We are waiting until we are able to find out a little more about this person," Peyroux said.

The organisation ae911truth.org has an online petition, signed by more than 16,400 people, calling on the US Congress to open an independent inquiry into the 9/11 attacks in September 2001. The group believes there is enough evidence to prove that the World Trade Centre's Twin Towers and Building 7 were destroyed by controlled demolition with explosives, not by the impact of the planes.

The Louvre branch opened in the former mining town of Lens in December.

• This article was amended on 12 February 2013. It originally stated in the penultimate paragraph that the ae911truth.org rejects the idea that Islamists flew two planes into the World Trade Centre. This has been corrected.