According to the French Interior Ministry, at around 09:30 local time (08:30 BST) a number of individuals got into the Louvre's Apollo Gallery via a window, which they broke.
Le Parisien reported the thieves entered via the Seine-facing facade, where construction is underway, and used a freight elevator to reach the gallery. After breaking windows, they reportedly stole nine pieces from the jewelery collection of Napoleon and the Empress, the paper said.
French authorities said forensic work is underway and a precise inventory of the stolen objects is being compiled, adding that the items have “inestimable” historical and patrimonial value.
Culture Minister Rachida Dati and Interior Minister Laurent Nuñez were on site with museum leadership.
Police have sealed off the museum and evacuated visitors. New arrivals were turned away and nearby streets were closed, according to the Interior Ministry.