A man who apparently shouted “this is for Syria” has been arrested after stabbing a commuter at a London tube station.

The suspect was wielding a large knife when he attacked the victim in the neck at Leytonstone station, leaving him seriously injured.

The Metropolitan Police are treating the attack as a “terrorist incident”.

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 sources.

Passengers ran from the scene as the incident unfolded.

White added initial evidence suggested it is a very “significant” incident.

Commander Richard Walton, who leads the Met’s Counter Terrorism Command, said: “We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant.

“The threat from terrorism remains at severe, which means that a terrorist attack is highly likely.”

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.”

Two others sustained minor injuries in the attack.