Trapped passengers were forced to smash the glass windows of the carriages to evacuate Clapham Common tube station after an emergency was announced.
Video footage from inside the station circulating on social media appears to show passengers rammed against the doors as people desperately attempt to pry them open.
British Transport Police told ITV News the issue is believed to have been caused by brake dust which can often be confused with burning.
It added there were no reported injuries and the incident has now been resolved.
London Fire Brigade said around 500 people left the train before firefighters arrived.
Transport for London (TFL) apologised for the issue and said it was investigating the incident.
Commuter Jake Sharp got caught up in the chaos and shared a terrifying video of the crisis.
Alongside the video, he wrote: "Stuck on the train at Clapham Common and carriages filling with smoke, doors won’t open if it wasn’t for workmen on the platform we wouldn’t have got out!
"TFL you should be ashamed! The response from station staff was horrific!"
One commuter who was on the Northern Line train that was evacuated described how passengers were left "panicking" and "banging to get attention" after what appeared to be smoke, but has now been confirmed to likely have been brake dust, started to fill their carriages.
She tweeted: "Strong smell of smoke and burning on the Northern Line, passenger alarm gets pulled and train stops at Clapham Common but doors won't open despite repeated shouting and banging to attract attention.
"Passengers panicking. Finally, lights go out and one door opens.
"Get out and multiple carriages have smashed windows and doors to get out/air. Not sure what happened but I'm not filled with confidence if there had been an immediate risk."
Meanwhile, another commuter said: "Doors were all sealed with us inside and other passengers banging and smashing windows to get out of carriages filling with smoke."
A British Transport Police spokesperson said: “BTP received a report of a faulty train at Clapham Common station shortly before 6pm this evening, and we are aware of a video showing passengers trying to leave the train.
"The issue is believed to have been caused by brake dust which can often be confused with burning.
"There were no reports of any injuries and the incident has now been resolved.”
A TFL spokesperson said: “We’re sorry for the distress caused at Clapham Common. The London Fire Brigade attended the station and confirmed there was no fire. We are investigating the details of this incident."
The station was evacuated due to the fire alert.
As of 9:40pm, Transport for London's website states that the Northern Line is experiencing "minor delays due to an earlier fire alert at Clapham Common."
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