The suspect was wielding a large knife when he attacked the victim in the neck at Leytonstone station, leaving him seriously injured.
Sky’s Mark White said: “A man in the main entrance hall by the ticket barrier, we are told, started attacking commuters leaving that station.
“Police officers burst on the scene are trying to subdue him by tasering him, it took several attempts to do that.”
Eyewitness reports said the alleged knifeman spoke of Syria, which has been confirmed to Sky News by police.
Passengers ran from the scene as the incident unfolded.
White added initial evidence suggested it is a very “significant” incident.
A Scotland Yard spokesman said: “Police were called at 7.06pm on Saturday 5 December to reports of a stabbing at Leytonstone Underground station. The male suspect was reportedly threatening other people with a knife.
“Met officers attended the scene.
“A man was arrested at 7.14pm and taken to an east London police station where he remains in custody.”
Three others are also thought to have suffered injuries.
SOURCE: Sky News