Euston Station was evacuated during rush hour after smoke was seen coming from a London Underground train.
Rush-hour commuters took to Twitter fearing a Northern Line train had caught fire at the hub.
They said they "could smell smoke" and "think there was a fire".
One person tweeted: "Smoke coming from train on Northern Line platform at Euston. Station evacuated, avoid!"
Another said: "Had a wild Monday morning today. #northernline train smelt increasingly of smoke, until we were told to get off it at #euston station. Rolls of smoke rolling of it, told to get back and then evacuated the whole station as the Firemen arrived."
Transport for London confirmed the station was closed while authorities dealt with a "fire alert."
And a spokeswoman from London Fire Brigade said they had three fire engines at the scene.
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She told Mirror Online: "We we called at 9.11am to reports of smoke issuing. Crews are investigating."
It led to delays and suspensions on the Underground network.
There was no service between Camden Town and Kennington via Bank southbound for some time.
But it is unclear if the alert had caused delays to National Rail services.
"Euston Station: Closed while we respond to a fire alert," reads a tweet on the official Northern Line account.
Videos on social media show scores of people heading for exits amid the evacuation.
Commuters said TfL staff told them to get off the train immediately at around 9.05am.
A spokesperson for the British Transport Police (BTP) said they were responding alongside the London Fire Brigade.
"We called at 9.10 and in the process of responding alongside the fire brigade," they said.
"It looks like part of a fire alert."
National Rail uses the 16 platforms at the station for services in and out the capital.
Euston Station in central London is the sixth-busiest station in Britain.
It handles more than 70 million passengers each year.
