Interior Minister Manuel Valls gave the initial toll of seven dead and said there also were "dozens of injured" following the accident at the Bretigny-sur-Orge station involving a train heading from Paris to the west-central city of Limoges.
The minister said that information on the number of dead and wounded was "constantly evolving" and that several train carriages were lying on their sides.
The Paris prefect's office said a "red alert" plan had been activated at 5:23 pm (2053 IST) following the accident.
"The train arrived at the station at high speed. It split in two for an unknown reason. Part of the train continued to roll while the other was left on its side on the platform," a police source told AFP.
The cause of the accident was not immediately known.
"It was not a collision and it was not a problem with the speed," a source with the SNCF national rail service told AFP.
The accident took place at 5:14 pm, the SNCF said, minutes after the "Intercite" train left the Paris-Austerlitz station.
Bretigny Mayor Bernard Decaux told newspaper Le Parisien that there was chaos at the station.
"Everyone is running in every direction, there is panic," he said. "It is an apocalyptic scene. We are trying to organise things."
Dozens of emergency and police vehicles had arrived at the scene, an AFP reporter said.
The accident occurred as many in France were departing for the start of their summer holidays.