Five people are in custody after an apparent failed bomb attempt in an upmarket district of Paris.
Police were alerted by a member of the public about suspicious activity in a building in the Porte d'Auteuil neighbourhood on Saturday.
Authorities later found and deactivated an explosive device.
Two gas cylinders had been discovered in the hallway of the building and two others on the pavement outside.
A mobile phone attached to the cylinders was being investigated as a possible detonator, reports suggested.
An inquiry by counter-terrorism prosecutors is under way following the alert in the affluent 16th district of the French capital.
Interior minister Gerard Collomb said one of those arrested was on a police terror watchlist and had been "radicalised".
Mr Collomb said the discovery of the device showed again that France was at an "extremely high" risk of terrorist attacks.
French politicians are set to vote later on a new counter-terror law designed to end the country's two-year state of emergency.
Authorities would be able to place people under house arrest, order house searches and ban public gatherings without needing the prior approval of a judge.
Human rights groups and UN experts have claimed the proposed legislation gives too much power to the state.
However, Mr Collomb argued that the Paris incident and a knife attack in Marseille on Sunday that left two women dead underlined the importance of tough new security laws.
The legislation was approved by the upper house Senate in July and is set to be backed in the lower National Assembly, where the ruling centrist En Marche party has a comfortable majority.
At least 12 planned attacks in France have been foiled since the start of the year, officials claim.
The country has been under a state of emergency since the gun and bomb attacks in Paris in November 2015, which killed 130 people and left more than 400 injured.