Paris police have arrested a man on the terrorism watch list after his car was found near the Notre Dame Cathedral with seven gas cylinders inside.
The Peugeot 607 was found with its hazard warning lights flashing and without number plates, police said.
One of the canisters, on the front passenger seat, was empty, and there were no detonating devices inside.
In November, 130 people died in Paris in a series of terror attacks carried out by so-called Islamic State (IS).
An associate of the car's owner, also known to police, was also arrested, French news agency AFP reported.
Some French media reports said several people had been arrested.
Documents with writing in Arabic were also found in the car, police said.
Officers speculated that the hazard lights may have been turned on to attract attention. "We think he may have been trying to carry out a test-run," one police official told AFP.
The car was found on the Quai de Montebello, just metres from the cathedral, along a stretch of the Seine riverside.
A state of emergency has been in place in France since the attacks in Paris.
At least 84 people were killed when a lorry was driven into a crowd in the southern city of Nice in July. The driver, Mohamed Lahouaiej-Bouhlel, was believed to have been inspired by IS.
Notre Dame is among Paris' top tourist destinations, attracting approximately 13 million visitors per year.