The suspected Tube station terrorist punched and kicked one of his victims unconscious before knifing him as he lay on the floor, a witness has claimed.
A 56-year-old man was left with "serious" stab wounds and another person was injured during the terror attack last night at Leytonstone Tube station in east London.
Salim Patel, 59, who runs the station shop and saw the attack unfold, said he heard shouts and saw the attacker beat up his victim before leaving him unconscious on the station floor.
He then allegedly walked back over to the stricken man, pulled out his knife and stabbed him in a sawing motion, Mr Patel said.
The knifeman reportedly said "This is for Syria" after he carried out the assault.
Mr Patel told the Press Association: "I was just at the shop when the train arrived and people came off.
"I heard some shouting, but sometimes drunk people shout. Then I saw the guy attacking the victim, punching him so hard. The victim was screaming 'please somebody help me. Help'.
"Then the attacker started kicking him on the floor. I think the victim was unconscious, he passed out. Then the attacker took a knife out and started stabbing him as he lay on the floor.
"The victim did not shout anything. I think he was unconscious. The attacker was saying something, I couldn't hear what."
Scotland Yard said officers arrived at the scene shortly after 7pm and within five minutes of the emergency calls, and a 29-year-old man was restrained and arrested
Graphic footage of the attacks posted online soon after the incident show a large pool of blood spattered on the station floor.
The video appears to show the knifeman wildly gesticulating and confronting several people in the station, before slashing a man in the throat area.
Several uniformed police officers then arrive on the scene and Taser the man, who falls heavily to the ground.
Terrified passengers, some in tears and others with children, ran out of the Tube station as the attack unfolded, locals said.
Detectives are searching a residential address in east London after the "violent unprovoked knife attack", which is being treated as a "terrorist incident".
The 56-year-old man was taken by the London Ambulance Service to an east London hospital where he remains in a stable condition.
A second man sustained a minor injury during the incident but did not require medical assistance, while a woman was threatened but not injured.
Commander Richard Walton, who leads the Met's Counter Terrorism Command, said: "As a result of information received at the time from people who were at the scene and subsequent investigations carried out by the Counter Terrorism Command (SO15), I am treating this as a terrorist incident.
"I am aware that there were a number of members of the public in the underground station who filmed the suspect on their mobile phones during and after the attack and I would urge them to come forward and speak to us and share with us the filming that they carried out.
"I would continue to 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."
John Cryer, MP for Leyton and Wanstead, said: "It is really shocking. It's an appalling thing to happen - it is to any community but Leytonstone is a very mixed community and people live together here very peacefully. It is quite a happy place to be."
The attack comes weeks after the Paris terror attack, which claimed the lives of 130 people, and just days after Parliament voted to extend air strikes and bomb Islamic State (IS, also known as Isil, Isis and Daesh) to Syria.