ARMED police arrested a 19-year-old man in London on Friday under counter-terrorism laws in connection with a suspicious package found on the Underground network that sparked a security alert.

“The man was arrested by officers from the Met’s Counter Terrorism Command, assisted by armed colleagues, in the street in Holloway Road” in north London, the police said in a statement.

London’s Metropolitan Police said a taser was used during the arrest at around 12.20pm (local time) on the busy shopping street, but that no shots were fired.

Footage posted on Twitter shows people going about their daily business as a heavy police presence descends on Holloway Road.

“Something was happening on Holloway Road earlier today,” Emi Koizumi says in the post.

The police said they had made the package safe after it was found on a train at Greenwich North station on Thursday. It was now being “forensically examined”.

The Telegraph reports details of the suspected plot emerged in a memo sent to serving military personnel on Friday, forwarding a warning from the Ministry of Defence that monitors terror threat levels.

It said: “This increase is in response to the discovery of a suspected viable improvised explosive device (IED) on a London Underground train yesterday,” according to the media outlet.

“It is unknown who placed the device and what their motivation was. Therefore it remains possible that the perpetrator may attempt to place further devices.”

The Press Association news agency quoted a witness saying the man had been walking along the street “normally” when he was arrested.

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“The armed police, I think five of them, ran behind him and put him on the floor,” Ali, 30, who did not want to give his full name, told PA.

“He was struggling not to get arrested, for like a good five minutes. They were shouting ‘armed police, don’t struggle’,” Ali was quoted as saying.

Margaret Mathurin, 54, told PA she was having a coffee when she saw the police swooping.

“They were plain-clothed police with masks, they had guns, everything. It was very unnerving,” she said.

The London Underground system was operating as normal but police said the public would see “more officers, including armed police, in and around transport hubs to provide reassurance”.

Counter-terror detectives opened an investigation on Thursday after the suspicious item was reported at around 11am (local time) at North Greenwich station, which serves The O2 entertainment complex.

The police said in a statement on Thursday that “a controlled detonation” of the package had taken place.

“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,” it said.

Since August 2014, the terror threat level in Britain has been classed as “severe” — the second highest level, meaning an attack is considered “highly likely”.

London mayor Sadiq Khan praised the “outstanding professionalism” of police and Transport for London staff.

“This situation was dealt with swiftly and safely, and no injuries resulted,” he said.

“Keeping Londoners safe is my highest priority. I am urging all Londoners ... to remain calm and vigilant at all times.”