A shopkeeper who witnessed the knife attack at Leytonstone Tube station has described seeing a victim begging for help on the floor as he was punched and kicked.
Police were called to the east London tube station on Saturday evening after reports that a number of people had been stabbed and a man was threatening others with a knife.
Scotland Yard said officers arrived at the scene within five minutes of the emergency calls and the 29-year-old suspect was restrained and arrested after police discharged a Taser.
Detectives are currently searching a residential address in east London after the "violent unprovoked knife attack", which is being treated as a "terrorist incident".
Graphic footage of the attacks later emerged online showing the knifeman appearing to slash a man in the throat, and blood pooled on the station floor.
Salim Patel, 59, who runs the station shop and saw the attack unfold, said: "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."