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This is the moment a suspected knifeman was tackled by police after being tasered following a horrific attack in a Tube station.

Graphic footage of police confronting the alleged attacker has emerged as stunned witnesses claimed he screamed "this is for Syria".

Police are now treating the attack as a terrorist incident.

Read more: Leytonstone stabbing victim speaks out

Commander Richard Walton, who leads the Met's Counter Terrorism Command, said:"We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant.

"The threat from terrorism remains at severe, which means that a terrorist attack is highly likely.

"I would appeal to anyone who was in the vicinity of Leytonstone underground station earlier this evening at around 7pm who saw anything suspicious to call the anti-terror hotline on 0800 789 321.

"Anyone who may have captured video or photographic footage of the incident is also urged to make contact with the Counter Terrorism Command via the Anti Terror hotline".

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One person is in a serious condition in hospital while another two are feared to have suffered minor injuries following the attack at Leytonstone Underground Station this evening.

Footage captured by a stunned passer-by shows the moment police confronted the suspect in the ticket hall of the east London station.

The man, who has not been named, is allegedly seen clutching a knife in his hands as officers keep him away from commuters.

Read more: Man arrested after commuter attacked in Tube station

In the footage, posted to Twitter, police officers surround him before he was tasered.

As the officers clamber on top of him, one onlooker shouts out "You are no Muslim".

In the ticket hall a large pool of blood is seen on the floor as commuters watch on.

A Metropolitan Police spokesman confirmed the force was called to the station just after 7pm

One person, who claims to have witnessed the attack, took to Twitter.

He said: "So as I was going to Leytonstone station was dressed to go to Christmas dinner with people from work.

"As I walked down I just saw a lot of people running but I ignored it and kept walking to get my train, but suddenly what I saw I couldn't believe my eyes and what I saw was I guy with a knife and a dead guy on the floor.

"I was so scared I ran for my life. After good 10-15 police came and got the guy and arrested him.

"And as he was coming out this is what he said "This is what happens when you f*** with mother Syria all of your blood will be spilled"."

Police have not confirmed the attack is linked to Syria or that anyone shouted anything to do with the wartorn country.

Horrific video from the scene shows a large pool of blood on the floor as stunned commuters run away.

Bloody footprints can be seen surrounding the large puddle of blood.

A spokesman said: "Police were called at 19:06hrs on Saturday, 5 December, to reports of a stabbing at Leytonstone underground station.

"The male suspect was reportedly threatening other people with a knife.

"Met officers attended the scene. A man was arrested at 19:14hrs and taken to an east London police station where he remains in custody.

"A Taser was discharged by one of the Met officers.

"Officers from British Transport Police are now dealing with the incident at the scene.

"We are aware of one man having sustained serious stab injuries. We await details of any other injuries."

A spokeswoman from London Ambulance Service said: "We were called at 7:09pm to reports of an assault at Leytonstone underground station.

"We sent a number of resources including our joint response unit, an incident response officer, an ambulance crew and London's Air Ambulance to the scene.

"We treated a man for stab wounds.

"He was taken as a priority to hospital escorted by the doctor from London's Air Ambulance."