Britain's Counter-Terrorism Command is investigating the stabbing of at least three people at a London Underground train station on Saturday (local time), which left a pool of blood in the ticket hall.
One man was arrested following the incident at Leytonstone station in the east of the capital.
"We are treating this as a terrorist incident," CTC leader Commander Richard Walton said in a police statement.
Amateur video footage circulating on the internet showed a pool of blood and bloody footprints at the tube station ticket gates.
Some commuters continued on their journey while others briskly jogged away from the scene.
Footage shows an officer shouting "Drop the knife!" and firing a Taser electric stun gun.
A later clip shows the man being pinned to the ground by police as passers-by are told to get back.
One man at the scene shouted: "You ain't no Muslim, bruv."
Police said they were called at 7.06pm to reports of a number of people being stabbed at the suburban station on the Central Line.
"The male suspect was reportedly threatening other people with a knife," Scotland Yard police headquarters said.
"A man was arrested at 7.14pm and taken to an east London police station where he remains in custody.
"One man has sustained serious knife injuries; these are not believed at this stage to be life-threatening. Two other people have sustained minor injuries."
Walton said: "We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant."