A car discovered laden with gas cannisters was to be used in an attack on a Paris railway station, police said on Friday, as a seventh person was arrested over the plot.
“An alert has been issued to all stations but they had planned to attack the Gare de Lyon on Thursday,” an interior ministry official said on Friday.
Francois Hollande, the French president, meanwhile said police had “destroyed” an Islamic State cell, but warned there were others.
Four people were arrested on Thursday over the discovery last week of the car on a side road leading to Notre Dame cathedral. The Gare de Lyon station is in the southeast of the capital, less than 3km from the cathedral.
Three women were arrested overnight in Boussy-Saint-Antoine, 30km south-east of Paris, while the boyfriend of one of the women was also taken into custody, bringing to seven the total number of arrests.
The interior ministry added that the youngest of the three women, 19-year-old Ines Madani, whose father owned the car, had written a letter pledging allegiance to the Islamic State group.
The teenager was already suspected by police of wanting to go and fight for Islamic State in Syria. Reports in French media suggested she also attacked and injured a police officer with a knife during her arrest.
The Peugeot 607 was found on Saturday night laden with seven gas cylinders, six of them full. Police sources said no detonator had been found in the car, though the vehicle also contained three large cans of diesel.
The car had no registration plates and was left with its hazard lights flashing.
“These three women aged 39, 23 and 19 had been radicalised, were fanatics and were in all likelihood preparing an imminent, violent act,” said Bernard Cazeneuve, France’s interior minister.
The car’s owner was taken into custody earlier this week but later released. He had gone to police on Sunday to report that his daughter had disappeared with his car, officials said.