A knife attack that wounded three people at an underground rail station in London was being investigated on Sunday as a "terrorist incident".
A police officer engaged the attacker and used a stun gun to make the arrest on Saturday night at the Leytonstone Station.
"We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant," Commander Richard Walton, who leads the UK capital's Counter Terrorism Command, said in a statement published online.
"The threat from terrorism remains at severe, which means that a terrorist attack is highly likely."
One man was seriously hurt in the incident, but his injuries are not believed to be life-threatening, the police said. The two other people were less seriously wounded.
Footage of the scene at Leytonstone underground station in east London quickly circulated on social media.
One video showed a pool of blood on the floor, people climbing ticket barriers and fleeing, and an agitated man being confronted by police.
Another clip showed the same man lying face down on the ground as officers restrained him.
