The Paris metro is one of the oldest urban transit systems in the world.
The operator of the Paris metro system on Wednesday apologised after hundreds of passengers on different trains were blocked underground in stifling temperatures at the height of the rush hour.
The RATP operator's chief executive Jean Castex, a former French prime minister, has asked for an internal investigation to determine the causes of the "exceptional incident", it said.
It said that five trains on the busy line 4 were stuck from 1725 GMT and had to wait underground between stations.
Social media users shared footage of passengers crammed inside the carriages and expressing concern about the wellbeing of babies and the elderly on board.
All the passengers were eventually evacuated, the RATP said, with social media users showing images of them walking through tunnels to the next station.
"The RATP presents its apologies and its deepest regrets for this incident and its consequences for passengers. It is doing everything possible to support them as best it can," the operator said.
Castex "has requested an internal investigation to determine the exact causes of this rare incident," it added.
The Paris metro is one of the oldest urban transit systems in the world, featuring in numerous iconic films.
But users increasingly complain that overcapacity and outdated infrastructure make commuting a daily trauma, also raising concerns over whether the system will be able to cope with the influx of visitors for the 2024 Paris Olympics.
Modernisation efforts are in progress and line 4 is being gradually automated to a system without drivers.
President Emmanuel Macron last year chose Castex, who was premier during much of the Covid-19 pandemic and has a reputation as a troubleshooter, as the chief of the RATP with the aim of fixing its problems.