The failure happened at around 20:10 (1810 GMT) between two stations on Line 1 in the French capital's upmarket Marais district, a spokesman for the operator RATP said.
Traffic didn't fully resume until 23:30.
Passengers needed to be evacuated from at least eight trains, with many walking to the nearest station in the dark along the tracks, AFP reported.
The fire service said five people were treated for heat exhaustion after being stuck underground as France is experiencing a weeks-long heat wave.
The incident on Line 1 -- which serves some of Paris' most popular landmarks including the Louvre, the Tuileries garden and the Champs-Elysees -- comes after tens of thousands of people were stranded at a train station in the capital following an electrical fire over the weekend.