LONDON -- A man stabbed three people at an east London tube station Saturday before police subdued him with a stun gun and made an arrest, London's Metropolitan Police said.
Police said they were treating the stabbing at Leytonstone station as a terrorist incident.
One man sustained serious injuries not considered to be life-threatening, police said. Two others suffered minor injuries.
The incident happened just after 7 p.m., police said. The suspect was taken to a London police station.
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 senior European counterterrorism official said this week that intelligence obtained by European security agencies indicates ISIS is aiming to attack the United Kingdom following the terrorist attack in Paris last month.
Videos posted to YouTube and on Twitter purport to show the aftermath of Saturday's London stabbing. A large pool of blood is on the ground near the exit gates.
Several show police confronting a man inside the station, near the exit, and yelling, "Drop it! Right now!"
They fire a stun gun at the man, who is wearing a gray top, tan pants and a black hat with ear flaps. But the man continues pacing in front of the officers and even lunges at them.
In one video, a woman can be heard saying, "I just want to get out of here." The scene is loud, with voices echoing throughout the station and police repeatedly shouting at people to get back.
There is more shouting before another pop is heard, apparently from the stun gun, and the man falls to the ground. Someone watching shouts, "Yes! Stupid idiot!"
Two police officers roll the man onto his stomach and handcuff him.
A man in the crowd shouts, "You ain't no Muslim, bruv! You're no Muslim, bruv! You ain't no Muslim!"