Passengers were evacuated from Holborn station in London's West End today amid reports of a fire.
The Central Line's Twitter account said the station was closed "while we respond to a fire alert".
It said trains were initially being held at the station while a "faulty train" was inspected.
Several people posted on social media that they heard bangs, followed by smoke and panic.
User Sophie Walker posted: "Holborn station evacuated, there's smoke and people are crying and freaking out ..."
Al Gwatkin, 43, from Chelsea, was getting off the Piccadilly line to transfer on to the Central line at Holborn when the incident happened.
"All of a sudden everyone came running out saying 'they've evacuated the station'.
"You don't know if it's terrorist-related. You're just like 'oh my God, get out of here'. There was panic down there. People were screaming," he said.
A member of staff outside the station said there had been a problem with a train and emergency services were investigating.
Two fire engines, a police van and an emergency response unit were parked outside the station.