A 19-year-old man has been arrested under the Terrorism Act after a suspicious item was found on the Jubilee line.
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 no 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."
This follows the incident at North Greenwich station on Thursday morning. Police were called after train staff reported finding a "suspicious item" on an eastbound Jubilee Line train, and the station was evacuated while specialist officers made it safe.
Scotland Yard said the item is currently being forensically examined, but no further details have been released.
The Met said: "Officers are keeping an open mind regarding any possible motive. They are not looking for anyone else in relation to this investigation at this stage."
The public were told to expect to see more officers, including armed police, in and around transport hubs.
The current threat level for international terrorism in the UK is severe - meaning an attack is "highly likely".
Mayor of London Sadiq Khan said: "Thanks to the outstanding professionalism of the Metropolitan Police, Transport for London staff and the British Transport Police, this situation was dealt with swiftly and safely, and no injuries resulted.
"I have been in constant contact with the Metropolitan Police commissioner and the TfL commissioner throughout the past two days.
"Keeping Londoners safe is my highest priority. I am urging all Londoners to check with TfL before they travel and to remain calm and vigilant at all times."