Network operator RATP was unable to determine the cause of the incident, forcing the shutdown of the entire line.
Police said on Wednesday that 3,200 to 3,800 people on at least eight trains needed to be evacuated in an operation which lasted more than two hours.
The RATP said it could not confirm the number of passengers affected by what it called “an extremely rare event”.
After long waits in the cars as temperatures — and tempers — rose, about 40 rescue workers and 150 RATP agents began helping people climb down from the cars and walk out along the tracks.
Five people required treatment for heat exhaustion during the evacuations, which came as France is being hit with a weeks-long heat wave.
Many passengers vented their frustrations on social media, posting videos of a chaotic evacuation and complaining about a lack of information about what has happening.
“No air conditioning, a real oven,” tweeted Assma Maad, a journalist at Buzzfeed France.
“I plan to add the French subway on Tripadvisor as Best Mixed Hammam and worst Escape Game in Paris,” a Twitter user identified as Celine Tran posted.
The RATP said that after 30 minutes of waiting, several alarms were pulled inside cars, resulting in power outages which cut off the air conditioning.
It also said hundreds of calls to the line’s control post had overloaded the passenger information systems, preventing it from updating travellers.