Four people have been treated for smoke inhalation following a fire on a Tube train at Oxford Circus in central London.
The station was evacuated shortly before 09:00 BST and trains did not stop at the station for nearly two hours.
Pictures on social media showed smoke billowing out from a Bakerloo line train sitting on a platform.
Transport for London (TFL) said the fire was caused by an electrical fault.
The station reopened and services resumed at around 10:50.
London Ambulance Service said it treated four people at Oxford Circus for smoke inhalation, two of whom were taken to hospital as a precaution.
BBC journalist Katie Silver was in the station at the time. She said: "I was coming off the Victoria Line but there was smoke as you walked past the Bakerloo Line and there was the smell of smoke and burning plastic.
"We were all being evacuated. Some people were running up the escalator but they had shut certain exits, although it was very orderly.
"When we got out there were a couple of people crying. I think they were scared."
TFL said a "comprehensive investigation" would take place.
Nigel Holness, London Underground's director of network operations, said: "The cause of the smoke was an electrical fault under one of the carriages, which resulted in a small fire that was quickly extinguished.
"Understandably this was a distressing incident for our customers, for which I give my sincere apologies."
Who is Inspector Sands?
If you are ever in a Tube station and hear the announcement "would Inspector Sands please go to the operations room immediately", you may find that you are asked to leave the station a few minutes later.
The message, which can be heard in the above video clip, is an automated message which is activated when the station fire alarm sounds.
TFL says the message gives them an opportunity to investigate why the fire alarm is operating.
It is a safety mechanism that has been agreed with the London Fire Brigade (LFB) and is the same system used in major national rail stations and airports in the UK.
LFB is continuing to investigate the cause of the fire.
It said it was called to attend at 08:47 and the station had been evacuated before fire crews arrived.
Tom Singer, who was at Oxford Circus station at the time, told the BBC's Victoria Derbyshire programme he saw a smoke-filled Bakerloo line Tube train coming into the platform.
"As we were waiting on the platform you could smell something coming in, as the train was coming in, and as it got closer the smell got stronger and stronger."
He added that as the train came in it was clear passengers had been moved from one of the carriages, and then when the train came to a stop people "came pouring out".
He said: "There were a lot of people with hands over mouths and coughing a lot coming off of the train.
"It looked like it was just one carriage that had the problem."
He said he also saw smoke coming from underneath the Tube train.