Anti-terror officers are questioning a man suspected of stabbing two men in a Tube station and declaring "This is for Syria", as police increased patrols at transport hubs to "identify and deter terrorism".
A 56-year-old man was left with "serious" stab wounds and another person was injured during the attack on Saturday night at Leytonstone 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, leaving him unconscious on the station floor, before walking back pulling out a knife and stabbing him in a sawing motion.
The knifeman reportedly said "this is for Syria", after he carried out the assault.
A 29-year-old man was Tasered by police and arrested just before 7.15pm, Scotland Yard said.
They have up to 36 hours to question the suspect before deciding whether to charge him.
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 attacker was saying something, I couldn't hear what."
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.
Detectives are searching an address in east London after the "violent unprovoked knife attack", which is being treated as a "terrorist incident".
Assistant Chief Constable Mark Newton of British Transport Police said yesterday: "The safety of the travelling public remains our top priority. In addition to our usual specialist response teams, last week we launched Project Servator, deploying even more highly-visible police patrols designed to identify and deter terrorism.
"Following the incident last night, we are now deploying still more officers in this role. They involve both uniformed and plain-clothed officers, supported by other resources, such as armed officers, police dogs, a network of CCTV cameras, and the thousands of rail staff we work alongside.
"We ask the public to remain calm and carry on using public transport as normal."