Emergency services are at the scene in South London with early reports suggesting several are injured amid widespread panic.
There are reports of people fleeing the scene screaming with a reporter saying people are “really badly burned” adding that “their hair was coming off”.
An onlooker said a “fireball flew down a carriage” with panicked commuters diving out an open door.
A picture believed to have been taken on a District Line train shows a burning plastic bucket.
The blast appears to have come from the contents of a Lidl bag left by the tube carriage door.
Train services through the area have been halted with commuters reporting they were informed of a “security incident”.
Police is investigating the incident.