Six people have been killed and more than 20 injured in a train crash at Bretigny-sur-Orge, south of Paris, French officials have said.
The intercity train had just left Paris and was heading towards Limoges when it derailed, crashing into a station platform at 17:14 (15:14 GMT).
Passengers were left trapped inside carriages.
French President Francois Hollande visited the scene and expressed his shock at the accident.
Six people died and 22 others were seriously injured, Mr Hollande told reporters. All those with serious injuries were now in hospital, he added.
Shortly after the crash, officials said as many as eight people were thought to have died.
BBC Paris correspondent Hugh Schofield said there was a strong possibility that the number of fatalities could rise again because some of the injured were very badly hurt.
The French president spoke of his "solidarity" with the families of those caught up in the accident, saying that the station would be closed for three days while investigations were carried out to determine what led to the derailment.
The rail company SNCF said 385 passengers were on board when the train crashed. The station platforms were crowded at the start of a holiday weekend.
Distressing accounts have emerged from the scene of the crash. British student Marvin Khareem Wone was on a train on another platform when the carriages of the intercity ploughed into the station.
"The train went off the railway; it just went on the platform and kind of flew in the air for a second and went upside down," he told BBC Radio.
"The first and the second coach were completely destroyed. I really thought no-one could survive that because it was completely mashed up. Everyone was crying and running everywhere. A woman was crying for her daughter who was still on the train."
Because of the damage to the station, he said ambulances could not reach the platform and the lift was not working.
Other media reports spoke of passengers being electrocuted and crushed.
"I saw many wounded women children trapped inside," Vianey Kalisa, who was waiting for his train from Bretigny to Paris, told AFP.
"People were screaming. A man had blood on his face. These are images of war," he said.
Bretigny mayor and local MP Michel Pouzol said four of the carriages were lying wrecked on the station platform.
He said the emergency services responded to a "red alert", put into action when many casualties are expected.
Guillaume Pepy, president of SNCF, described the accident as a "catastrophe" and explained that the train's third and fourth carriages derailed first and the other coaches followed.
A witness named only as Ophelie told the I-Tele news channel everyone was shocked and some people on the platforms were among the injured.
French transport routes were particularly busy at the time of the crash due to the run-up to a holiday weekend marking Sunday's Bastille Day.
"There were plenty of people on the platforms because of it being a holiday weekend," said Jean Paul Huchon, Socialist president in the IIe de France area around the capital.
The BBC's Hugh Schofield says that in May, SNCF warned passengers that points work was being carried out in the area, where intercity and commuter lines dovetail.
It is not clear if the earlier points problems are connected with Friday's crash, he says.
"We can't confirm anything without proof. I'll speak when it's clear. I will be completely transparent but right now everything must be done to help the victims," Mr Pepy told reporters in Bretigny.
Judicial and transport ministry inquiries as well as an SNCF investigation have been launched.
The train had left Paris-Austerlitz station at 16 :53 local time and was due to arrive at Limoges-Benedictins at 20:05, the company said in a statement.