The 15-year-old entered a Monoprix supermarket in a shopping centre in Boulogne-Billancourt at around 6.30pm.
He was armed with a fake American assault rifle and an automatic pistol, the local police commissioner said in a statement.
What is purported to be a witness video has emerged, reported France TV Info, showing one of the fake guns after it was confiscated by authorities (see below).
“We didn't know what was happening, everyone just started running,” one witness told Le Parisien newspaper.
“Everyone was in a panic, people were screaming,” another told BFM TV.
Police quickly arrested the teenager, and brought calm to the scene, although emergency services were called out to help at least one woman who had a panic attack, reported France TV Info.
Officers later said that the teen had been threatening customers, and was under the influence of alcohol at the time.
It remains unknown why the teen – who has been referred to as a “star student” by the French media – would pull such a stunt.
Monday saw a second gun incident in a shopping centre, this time in the south city of Avignon. An armed person killed a man and injured another in what local media reported was a robbery gone wrong.
France and especially Paris remain tetchy after terrorists armed with kalashnikovs killed 130 people in an indescriminate massacre in November.
The nation remains in a state of emergency, which France's President Francois Hollande plans to extend.