Police find two cylinders of gas in a building and two on the pavement, with the country on alert after Sunday's attack in Marseille.
The Paris prosecutor has opened an anti-terror investigation after the discovery at the weekend of a homemade bomb at a building in Paris, and five people have been arrested, sources close to the case said Monday.
Police attention was drawn to two cylinders of gas in the hall of a building in the city's posh 16th arrondissement and two others on the pavement early on Saturday.
They found a mobile phone linked to the cylinders, which appeared to be "a device to ignite" the cyclinders, the sources said.
A resident alerted police after finding two of the gas cylinders at around 4:30am on Saturday in his building in the Porte d'Auteuil neighbourhood, a source close to the probe said. Police then found the other two cylinders outside the building.
According to another source, five suspects were being held in custody Monday night.
The terrorist threat remains high in France, which has been hit with a series of militant attacks since January 2015.
The latest attack was on Sunday in Marseille, when a knifeman stabbed two women to death at the main train station shouting "Allahu Akbar" (God is greatest). He was shot and killed by soldiers.
The stabbings bring to 241 the number of people killed in militant attacks in France.
On September 12, French Interior Minister Gerard Collomb said that 12 planned attacks had been foiled since the beginning of the year.