A packed passenger train has derailed and crashed into a station outside Paris on the eve of a major holiday weekend.

At least seven people are believed killed and dozens have been injured, authorities say.

The Friday crash at Bretigny-sur-Orge station was the deadliest in France in years.

Some cars slid toward the station itself, crushing part of the metallic roof over the platform.

Images from the scene on French television showed gnarled metal and shards on the platform, and debris from the crash clogging the stairwell leading beneath.

Officials didn't comment on reports some passengers may still be trapped on the train.

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 travelling.

The head of the SNCF rail authority, Guillaume Pepy, called it a "catastrophe".

The cause of the crash is under investigation.

Two train cars, Nos. 3 and 4, initially derailed then knocked the other cars off the track, Pepy said.

"Some cars simply derailed, others are leaning, others fell over," he said.

Interior Minister Manuel Valls said at least seven people were believed dead and several dozen injured but he added that the casualty toll was "in constant evolution".

The SNCF said the train was carrying about 385 passengers when it derailed on Friday evening at 5.15 pm local time and crashed into the station at Bretigny-sur-Orge, about 20 kilometres south of Paris.

The train was headed from Paris to Limoges, a distance of 400 kilometres, and was about 20 minutes into what would have been a three-hour journey.

The accident came as France prepares 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 on summer vacation.