The stabbing of three people at a London tube station by a man wielding a large knife is being treated as a "terrorist incident", British police have said.
The suspect apparently shouted "this is for Syria" before attacking members of the public near the ticket barriers at Leytonstone station.
One man was stabbed in the neck, leaving him seriously injured. He is believed to be aged in his 50s and his condition is reported to be not life-threatening.
Two other people suffered minor injuries.
The suspect, understood to be a 29-year-old man, is being questioned by police in east London after being taken into custody.
Video footage shows police officers trying to subdue the suspect with a taser several times before they successfully bring him to the ground.
Sky News reporter Mark White said: "A man in the main entrance hall by the ticket barrier, we are told, started attacking commuters leaving that station."
Passengers ran from the scene as the incident unfolded.
One eyewitness, who was outside the station as events unfolded, said: "I could hear people pleading for help... and all of a sudden people started running out of the station."
Commander Richard Walton, who leads the Met Police'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."
Commenting on the attack, British Labour party leader Jeremy Corbyn MP wrote on Twitter: "The stabbing in Leytonstone is absolutely shocking. My thoughts are with the victim and his family."
Downing Street said it was "monitoring the situation closely as further details emerge" in the ongoing police investigation into the attack.
