A 19-year-old man has been arrested under the Terrorism Act after a suspicious device was found on a Tube train.
The suspect was seized in the street on Friday in an armed operation in Holloway Road, north London.
The Metropolitan Police said a Taser was discharged during the arrest but not shots were fired.
In a statement , the force said: "The 19-year-old man was arrested on suspicion of the commission, preparation and instigation of terrorism acts, under Section 41 of the Terrorism Act 2000. He has been taken to a London police station where he remains in custody."
The police swoop came after a controlled detonation was carried out on the device found on a train at North Greenwich station, close to the O2 complex in south east London on Thursday morning.