London police say the actions of a knife-wielding maniac who attacked three London commuters while shouting "this is for Syria" amount to terrorism.

"We are treating this as a terrorist incident," Commander Richard Walton, of the Metropolitan Police's Counter-Terrorism Command, told the media.

"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."

The immediate aftermath of the disturbing attack, which occurred at Leytonstone Station shortly after 7pm local time (around 6am AEST), was captured on video by an acquaintance of Facebooker "Kibzzy Free-gaza Rahman", who uploaded the shocking footage just hours ago.

The vision shows the suspect, who reportedly shouted "all your blood will be spilled" during the attack, coming face to face with two unarmed police as travellers hurl abuse at him.

"Taser! Taser!" one of the police officers can be heard shouting.

"Yes! Stupid idiot," a bystander shouts at the attacker as he goes down.

"You're not Muslim bro!" another eyewitness yells at the man as he lies on the ground.

"You ain't no Muslim."

According to the Met, the man whose throat was cut is in a critical condition; the state of the other two victims is not yet clear.

"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," police said in a statement issued earlier.

"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..."

Like several other great cities of the northern hemisphere, London has not been immune from terror attacks and has long faced the problem of the radicalisation of disaffected youth all too ready to commit sickening acts of violence under the pretence of righteousness.