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Police believe a terror plot to target the London Underground has been foiled after what they suspect to be a viable bomb was found on a train - planted by a 'lone wolf' terrorist who may have had other devices.

A 19-year-old suspect was seized in the street on Friday lunchtime in an armed operation - where police were forced to deploy a Taser but no shots were fired - in Holloway Road, north London.

The swoop came after a controlled explosion was carried out on a suspicious item found at the busy London underground station - which services The O2 arena music venue - on Thursday.

Police are understood to believe the alert was the result of a 'lone wolf' terrorist planting the device.

And in a note the MoD said it is believed there may be "other devices" designed by the perpetrator.

Reports say that the suspect "item" had passed under "high-level target" London landmarks like Westminster and Canary Wharf on the Jubilee Line before being spotted at North Greenwich.

A worker found an abandoned bag “full of wires” - and carried it through the train to the driver’s compartment - are also unconfirmed.

ASLEF train driver's union district organiser Finn Brennan told Mirror Online that the package - believed to be a bag - was initially mistaken for lost property and passed to the driver .

He added: "The driver contacted control at North Greenwich as soon as he became suspicious of it".

The teenager was arrested at around 12.20pm today in broad daylight - in front of scores of shoppers - in an underpass very close to Holloway Road station.

One witness said the 19-year-old man was walking through the underpass before being dragged to the ground by masked and armed anti-terror officers.

By 3.30pm there was still a heavy police presence but the road had been cleared for cars and pedestrians to pass.

(Image: Sky News)

The Metropolitan Police confirmed a Taser was discharged during today's arrest but no shots were fired.

Pictures taken by bystanders at the scene show a large area outside a branch of Costa Coffee and Sainsbury’s supermarket which are sealed off by police.

Costa Coffee customers said they watched in fear as the dramatic scenes unfolded in front of their eyes.

Community support worker, Margaret Mathurin, had just sat down with her coffee when armed police swooped.

The 54-year-old said: "It was very unnerving. It was just after 12pm and there were suddenly police vans and cars everywhere.

"They stopped all the traffic and jumped out. It was bizarre and all happened so quickly."

Margaret said police closed the road from McDonald's to London Metropolitan University.

She said: "They lined up with their guns and blocked the road. Nobody was allowed to go through but I was watching from Costa's window.

"I haven't moved from this spot since. The whole thing was unfolding before me.

"Although it was unnerving to see armed police on the streets of London I have to say it looked like they had control of the situation and knew what they were doing."

Another customer, a student called Jane, said: "There must have been at least 10 police vans here - maybe more."

Emi Koizumi, 42, described the terrifying moment armed police swarmed the road as he walked to get lunch at a nearby Dirty Burger restaurant.

She said: "We saw loads of police, including armed and also plain clothed police with their faces covered.

"The plain clothed police ran off down the street towards Highbury Corner right as we walked up.

"No one would tell us what was going on under the railway bridge."

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."

Police confirmed last night that counter-terrorism officers had launched an investigation.

Police were called shortly after 10am yesterday morning after rail staff reported finding the item on a Jubilee line train at North Greenwich underground station.

The line also runs into the Canary Wharf financial district - and hundreds of people had to be evacuated.

(Image: Sky News)

It said a controlled detonation was carried out to make the item safe. Unconfirmed reports say that this was carried out by the Army's bomb squad.

Officers from London's Counter Terrorism Command took the lead in the investigation, police said.

The North Greenwich underground station was evacuated - and disruption to the Jubilee line occurred for the rest of the afternoon.

Britain is on its second-highest alert level of "severe", meaning an attack is considered highly likely.

Extra police officers have been drafted onto mainline trains and the tube network today.

In a statement issued by ASLEF to its members it said that - while it was not yet clear if it was a viable or fake device - that they "will continue, calmly and professionally doing the job of keeping our city moving".

Mayor of London Sadiq Khan urged Londoners to remain calm in the wake of the arrest.

And he said that security around transport hubs would be beefed up in the wake of the alert.

(Image: Sky News)

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.

"Over the next few days you will see more officers in and around transport hubs to provide reassurance about public safety."

- Police are appealing to anyone with any information to call the Anti Terrorist Hotline on 0800 789 321.