Witnesses said the site of the crash resembled "a war zone", with one survivor describing walking over a decapitated body to escape an overturned carriage.
Rescuers were searching into the night for passengers who might be trapped in the wreckage.
The train was a regional service heading from Paris to the west-central city of Limoges. It derailed as it passed through the station at Bretigny-sur-Orge, about 25 kilometres (15 miles) south of Paris.
Prime Minister Jean-Marc Ayrault, speaking from the accident site late on Friday, told reporters: "The toll is currently six dead, 30 injured, of whom eight are in a serious condition."
The injured were all being treated in local hospitals, he added.
The head of the SNCF rail service, 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 added.
Clearly shaken, he expressed the rail company's "solidarity with the victims and their families".
"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. The rail service, judicial authorities and France's BEA safety agency would each be investigating.
Passenger Marc Cheutin, 57, told AFP he had to "step over a decapitated person" after the accident to exit the carriage he had been travelling in.
"Shortly after departure, just as I was getting into my book, we felt a first shock that shook the carriage I was in.... Then there was a second shock and the carriage lifted up, then a third and a fourth and the carriage went over on its side," he said.
A witness who had been waiting for a train at the station, Vianey Kalisa, told AFP: "I saw a lot of wounded people, women and children trapped inside (the carriages).
"I was shaking like a child. People were screaming. One man's face was covered in blood. It was a like a war zone."
In the immediate aftermath of the accident, dazed survivors, some in blood-stained clothing, stood near mangled train wagons and the crushed remains of a station platform.
Director of security at SNCF Alain Krakovitch praised the quick reaction of the train driver.
"He felt jolts entering the station and applied the regulation measures in the space of a few seconds, which is to say a radio warning signal and a light warning signal," thus stopping all traffic in the area.
His quick thinking avoided any collision with approaching trains, Krakovitch told reporters.
Earlier in the evening French President Francois Hollande visited the scene and met officials.
He confirmed that three investigations were underway, but added: "We should avoid unnecessary speculation. What happened will eventually be known and the proper conclusions will be drawn."
Officials said the derailment happened at 5:14 pm (1514 GMT), minutes after the intercity train left the Paris-Austerlitz station.
"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.
But a source with the SNCF told AFP: "It was not a collision and it was not a problem with the speed."
Transport minister Frederic Cuvillier, who also visited the crash site, said the train had been travelling at 137 kilometres an hour (85 miles per hour) at the time of the crash.
That was below the 150 kilometer-an-hour limit for that part of the track.
Some 300 firefighters, 20 paramedic teams and eight helicopters were deployed to treat casualties at the scene and airlift the most seriously injured to nearby hospitals.
In total, 192 people were treated by emergency services, officials said. There were 385 passengers on the train.
The accident occurred as many in France were departing for the start of their summer holidays ahead of Bastille Day on Sunday.
In Brussels, EU Commission chief Jose Manuel Barroso expressed his condolences.
"The derailment of the train casts a pall over the holiday departures as your country is preparing its national day," he said in a message to Hollande.
The derailment was France's worst rail accident since an SNCF commuter train crashed into a stationary train at Paris's Gare de Lyon terminal in 1988, killing 56 people.