People stand in front of Delacroix's masterpiece "Liberty Leading the People" on display in Lens, on December 4, 2012
French police were holding a mentally "unbalanced" woman after an attack on one of France's most iconic paintings, Delacroix's "Liberty Leading the People".
The incident happened at the recently opened northern satellite of the Louvre museum, in Lens, northern France, when a visitor scrawled on the painting with a black marker. It was not believed to have caused any permanent damage.
Prosecutors in the town said that after having received a psychiatric report on the 28-year-old woman, they would probably have her committed on Saturday.
The attack on the painting happened on Thursday evening, just before the 6:00 pm closing time. A security guard apprehended the woman with the help of a visitor to the museum.
The attacker scrawled "AE911" on the painting.
Architects & Engineers for 9/11 Truth (AE911Truth) is a US group that supports a conspiracy theory that New York's World Trade Center collapsed as the result of a controlled demolition after the September 11 attacks of 2001.
Local prosecutor Philippe Peyroux said: "We are leaning towards hospitalising her immediately in a psychiatric facility given that the psychiatrist appointed by the prosecutors concluded that she is not criminally responsible."
The Louvre said specialists had already been able to completely remove the approximately 30-centimetre (12-inch) mark on the bottom right of the painting.
"The integrity of the work has not been affected, as the inscription was superficial and remained on the varnished surface without reaching the layer of paint," the museum said in a statement.
In the United States the founder of the AE911Truth group, Richard Gage, told AFP: "I was shocked and horrified to learn of this senseless act of vandalism.
"I sincerely hope that this unbalanced person is not in any way associated with our numerous volunteers in France."
The painting by Eugene Delacroix commemorates France's July Revolution of 1830.
It shows a bare-breasted woman personifying Liberty leading the people forward over the bodies of the fallen, holding the French tricolour in one hand and a bayoneted musket in the other.
The wing housing the painting was closed Friday but expected to reopen Saturday.
The woman would have faced up to seven years in prison and a 100,000 euro ($135,000) fine if found criminally responsible and convicted of defacing a cultural object.
Lens mayor Guy Delcourt said the woman had "told security, in a rather incoherent manner, that she wanted to put her mark" on the painting.
Prosecutors did not release the woman's identity, but said she was unemployed, had a master's degree and did not have a criminal record.
The Louvre opened the new satellite branch in Lens -- a former mining town plagued by high unemployment -- in December in a bid to revive the region and boost tourism.
Officials said security would be strengthened at the museum following the incident, with more guards expected to be posted.
In 2009 a woman threw a mug of tea at the Mona Lisa at the Louvre's main museum in Paris but it bounced off the protective glass around the painting.
In December a Polish man was jailed for two years in Britain for defacing a mural by US artist Mark Rothko at London's Tate Modern gallery.