Witnesses said the site of the crash resembled "a war zone", with one describing walking over a decapitated body to exit a carriage that had been thrown on its side.
Rescuers were searching late yesterday for passengers who may still be trapped inside overturned carriages.
The train, a regional service heading from Paris to the west-central city of Limoges yesterday, hurled off the line as it passed at high speed through the station at Bretigny-sur-Orge, about 25 kilometres south of Paris.
Interior Minister Manuel Valls had earlier spoken of seven dead and "dozens injured," warning that the death toll was likely to rise.
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Some 300 firefighters, 20 paramedic teams and eight helicopters were deployed to the area. All Paris region hospitals were put on alert to deal with the injured.
In total, 192 people were treated by emergency services, officials said.
The head of the SNCF national railway, Guillaume Pepy, told reporters at the scene that six carriages had derailed during the accident. The train's third and fourth carriages derailed first and the others followed, he said.
"Rail catastrophes are something that upset everyone and all of those who are committed to our national rail service," he said.
"We don't yet know the reasons for this derailment," Pepy said, adding that investigations would be carried out by the rail service, judicial authorities and France's BEA safety agency.
After travelling to the scene and meeting with officials, President Francois Hollande confirmed that three investigations had been launched "to determine what happened at this station".
"We should avoid unnecessary speculation, what happened will eventually be known and the proper conclusions will be drawn," Hollande said.