Commuters watched in horror as a young man was struck by a London Underground train as he bent down to reach his bag.
The man, believed to be in his 20s, is in a critical condition after the incident on Friday morning at Stockwell Underground station.
The station, which also serves the Victoria line, was shut while medics tended to the passenger who was later taken to hospital.
Chris Duncan tweeted: "Was on this train. Awful. Man hit in the head by oncoming Tube train in horrific accident."
Isabelle Zonderland told London's Evening Standard: "I was on the tube when it happened. He got hit only metres from our carriage.
"When we were let off the tube I saw him lying there. Someone who I believe was on the platform was checking for a pulse. His face was covered in blood."
Was on this train. Awful. Man hit in the head by oncoming Tube train in horrific accident at Stockwell station http://t.co/jLqvTwasnR— Chris Duncan (@ChrisDuncanJT) April 10, 2015
Actor Raj Lucas told the Standard: "I was about midway on the platform and the tube was pulling in and suddenly I heard screams from the start of the platform.
"The people closer to the incident (who saw it) were visibly very distressed."
Nick Brown, London Underground's chief operating officer, said: "Just before 9am a passenger on a Northern line platform was struck by a train entering Stockwell station.
"Our emergency response team and the London Ambulance Service attended the scene. We are investigating the incident to understand exactly what happened but we have already established that while the station was busy, the platform was not overcrowded."
Today's accident comes after last month's tragic incident in which Londoner Nick Mann, 35, died of severe head injuries when he attempted to save his 32-year-old brother Robert from being struck by a train at Old Street Tube station.
Robert Mann remains in hospital.
The woman ended up falling into the gap between two carriages of the Northern line train and was taken to hospital with serious arm and head injuries.
The incident is being investigated by the Rail Accident Investigation Branch.