Police are continuing to question a teenager arrested by counter-terrorism officers after a device was discovered on a Tube train.
Armed police swooped on the 19-year-old suspect on Friday afternoon in Holloway Road, north London, after the find sparked a major security scare.
A Taser was discharged during the arrest but no shots were fired, the Metropolitan Police said.
The operation came after a controlled detonation was carried out on the suspicious item at North Greenwich station, close to the O2 complex in south-east London on Thursday morning.
Scotland Yard 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 Daily Telegraph has reported that a memo said to have been passed to serving military personnel from security officials stated a "suspected viable improvised explosive device" had been uncovered on the Tube.
Written before the arrest, it added that the threat level to transport in London had been raised to "severe" and that "it remains possible that the perpetrator may attempt to place further devices".
The arrested man was white, bearded and wearing a hoodie, a witness said.
Ali, 30, who did not want to give his full name, said the man had been walking along the street "normally" when police descended.
He said: "The armed police, I think five of them, ran behind him and put him on the floor."
Ali said he did not see the man Tasered but saw the Taser on the floor.
"He was struggling not to get arrested for like a good five minutes. They were shouting, 'Armed police, don't struggle!'. Everything happened so quick."
He described the arrested man as in his early 20s, white and bearded and wearing a hoodie, a long jacket, jeans and trainers.
"He did not look like he had had a shave for a long time," he added.
Another man, who did not want to be named, said at around 12.15pm he saw police cars arrive on the London road.
He added that he heard them shouting "Go, go, go!".
"They jumped out of the normal cars and put masks on and they had big guns," he told the Press Association.
"There was a person on the floor and there were 10 people around and on top and blocking him."
British Transport Police was called to the station shortly after 11am on Thursday after train staff reported finding a "suspicious item" on a train travelling eastbound on the Jubilee Line.
Officers evacuated the station and it remained shut for several hours as specialist teams worked to make the item safe.
No details about the device have been officially released but reports suggested it was handed to a Tube driver and contained wires and possibly a clock.
The device is currently being forensically examined and investigators are awaiting the results of the tests, police added.
Scotland Yard 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."
Commuters were told to expect to see more officers, including armed police, in and around transport hubs while patrols by BTP officers on the underground and at stations are in place.
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."
North Greenwich station reopened on Thursday night and a performance at the O2 by the band Nickelback went ahead as planned.
:: Police are asking anyone who saw anything suspicious on the train or anywhere else to call the anti-terrorist hotline on 0800 789 321.