"Fire in progress under the Pont National [National Bridge] at #Paris13. The @PompiersParis [the capital's firefighter service] are intervening with the security forces," the prefecture tweeted.
Traffic is cut off on the Porte de Bercy interchange of the bridge in both directions for security reasons, it added. The Francois Mitterrand Library is located a few blocks away.
Photos and videos from the scene have been shared online.
The fire department urged people to avoid the area but said the bridge is not at risk of collapse.