The Louvre's Lens branch closed two galleries on Friday after a woman defaced its main attraction, Eugene Delacroix's Liberty Leading the People.
The 28-year-old woman scribbled "AE911" in permanent marker on a foot long section at the bottom of the painting.
"AE911" is a shortened form of "Architects & Engineers for 9/11 Truth", a group whose website says they are calling for an inquiry into the World Trade Centre attacks to see whether they were destroyed by explosives. She was immediately seized by a guard and another visitor, and is being questioned by police.
"She appears to be a person with mental health problems," said the local prosecutor, Philippe Peyroux.