A knife attack on three people at a Tube station is being treated as a terrorist incident, police have said.
The man, who was said to be wielding a machete, reportedly said "This is for Syria" after he launched the attack at Leytonstone station in east London.
Shocking video footage posted online shows a large pool of blood spattered across the floor of the ticket hall as a man is seen threatening other passengers in the station.
One man has suffered serious knife wounds while two other people sustained minor injuries in Saturday's attack.
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 spokeswoman for 10 Downing Street said: "There is an ongoing police investigation into tonight's attack in London. We are monitoring the situation closely as further details emerge."
In the video, the knifeman approaches and gesticulates at two people in the station.
As they approach him he lashes out with his knife and appears to slash one of them in the neck area.
Moments later a uniformed police officer arrives on the scene and fires his Taser at the man.
In another video of the incident posted online, the man is pinned down on the floor by two police officers as another man can be heard shouting out to him: "You ain't no Muslim bruv."
Police said they were called just after 7pm to reports that a number of people had been stabbed at the station and a man was threatening people with a knife.
One man received serious knife injuries which are not believed to be life-threatening and two other people sustained minor injuries, Scotland Yard said.
A man was arrested just before 7.15pm and taken to an east London police station where he remained in custody.
Mr Walton appealed to anyone who was near Leytonstone Underground station at around 7pm and who saw anything suspicious to call the anti-terror hotline on 0800 789 321.
"Anyone who may have captured video or photographic footage of the incident is also urged to make contact with the Counter Terrorism Command via the Anti-Terror hotline," he said.
Commenting on the attack, Labour leader Jeremy Corbyn MP wrote on Twitter: "The stabbing in Leytonstone is absolutely shocking. My thoughts are with the victim and his family."