An electrical fire in the southern suburb of Issy-les-Moulineaux has nearly shut down departures at Montparnasse, and has forced state rail operator SNCF to reroute some arriving trains to Austerlitz, another main station in the capital.
SNCF has warned passengers of significant delays and potentially cancelled trains.
Suite à l'incendie d'un transformateur externe à SNCF, les plans de transport en gare Montparnasse sont modifiés : le nombre de trains sera réduit et d'importants retards sont à prévoir. @SNCF invite ses voyageurs à reporter dans la mesure du possible leurs voyages.SNCF (@SNCF) 27 juillet 2018
"Our technical teams are dealing with the situation," SNCF added in a tweet.
Photos and videos posted on Twitter showed a massive plume of smoke rising into the skies from an electrical centre in Issy-les-Moulineaux. The centre is operated by RTE, France's national electricity transmission system operator.
SNCF official Patrick Jeantet said the electrical centre hit by the fire was a crucial site which controls Montparnasse's safety installations.
The blaze, just nextdoor to Microsoft's main French offices, prompted an evacuation of 2,500 people and caused a power cut to thousands of homes in nearby suburbs, authorities said.
The incident came in the midst of a heatwave, further adding to travellers' woes, as temperatures in Paris soared to 35 degrees Celsius on Friday.
"Those who can postpone their journeys should do so," Alain Krakovitch, a senior official at rail operator SNCF, told reporters at Montparnasse.
The shutdown came just as many travellers were heading Friday to the beach or the countryside to start their August vacations.
Montparnasse station was the scene of similar transport chaos almost exactly a year ago, when a giant signalling failure caused three days of heavy disruption.