The crash at Bretigny-sur-Orge station was the deadliest in France in years, and President Francois Hollande rushed to the scene after abandoning plans in the capital.
Some of the train cars slid toward the station, crushing part of the metallic roof over the platform. Images from the site on French television and on Twitter showed gnarled metal and shards on the platform, and debris from the crash clogging the stairwell leading beneath the platform.
Some 300 firefighters, 20 medical teams and eight helicopters were deployed to get survivors out of the wreck, according to the Interior Ministry.
Hollande praised "the mobilization of the emergency services," and reached out in "solidarity with the victims' families." He added that an inquiry has been launched to determine the cause of the accident, which left around 190 people injured or in treatment for shock.
Ben Khelifa, a 20-year-old accounting apprentice whose commuter train was on the adjacent track, told The Associated Press that the derailed train "was unrecognizable.
"There was nothing but metal scraps," he said. "The train just collapsed, just like that, on its side... There was blood."
He added that he was one of a number of passengers in the adjacent train that went to help pull trapped survivors out of the wreckage. "People were screaming, people were asking where their children were," he said.
Another witness, Bazgua El Mehdi, 19, told Le Parisien newspaper: "I heard a loud noise. A cloud of sand covered everything. Then the dust dissipated. I thought it was a freight train, but then we saw the first casualties ... Many passengers on the (train) were crying."
It was unclear whether all the casualties were inside the train, or whether some had been on the platform, or how fast the train was traveling. The head of the SNCF rail authority, Guillaume Pepy, called it a "catastrophe."
The train's third and fourth cars initially derailed, which then knocked the other cars off the track, Pepy said. "Some cars simply derailed, others are leaning, others fell over," he said.
The Interior Ministry said six people died in the crash and nine were in critical condition. Earlier, Interior Minister Manuel Valls had said seven people died.
The SNCF said the train was carrying about 385 passengers when it derailed Friday evening at 5:15 p.m. (1515 GMT; 11:15 a.m. EDT) and crashed into the station at Bretigny-sur-Orge, about 20 kilometers (12 miles) south of Paris.
The train was headed from Paris to Limoges, a 400-kilometer (250-mile) journey, and was about 20 minutes into the scheduled three-hour journey.
The accident came as France is preparing to celebrate its most important national holiday, Bastille Day, on Sunday, and as masses of vacationers are heading out of Paris and other big cities to see family or for summer vacation.
All trains from Paris' Gare d'Austerlitz were suspended after the accident.
A passenger speaking on France's BFM television said the train was going at a normal speed and wasn't meant to stop at Bretigny-sur-Orge. He described children unattended in the chaotic aftermath.
AP writer Thomas Adamson contributed to this report from Paris.