Two women were killed by a knifeman who was then shot dead by soldiers in Marseille’s main railway station.
French police warned people to avoid Saint Charles station, tweeting that an operation was under way.
Soldiers and police took up positions outside the station, which was evacuated.
One woman was stabbed to death and the other woman’s throat was slit, source said.
The assailant was shot dead by soldiers who were patrolling inside the station.
The Paris prosecutors’ office said a counter-terrorism investigation has been opened.
No further details were immediately given, including the motive for the attack.
Interior Minister Gerard Collomb tweeted that he would travel to the scene.