Terrified passengers, including many children, were forced to run for their lives as a man wielding a machete allegedly stabbed a man in a London Tube station.
Video posted on Twitter shows the moment the alleged knifeman was Tasered by a police officer as pools of blood mark the floor in the Leytonstone Underground station.
A Scotland Yard spokesman confirmed police were called to the east London train station after reports of a stabbing in the ticket hall.
"We are treating this as a terrorist incident," Richard Walton, who leads the London police's Counter Terrorism Command, said in a statement.
"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."
3 injured in knife attack at London tube station. Police treating as terror after man yelled 'This is for Syria.' pic.twitter.com/EbdPy0mxCF
— Hugh Whitfeld (@hughwhitfeld) December 5, 2015
Witnesses have reported the man screamed 'this is for Syria' and 'all of your blood will be spilled' before allegedly slashing a person's throat and attacking three others.
He then turns on unarmed police officers, waving the large knife at them before lunging in their direction.
A spokeswoman from London Ambulance Service said the man with his throat slashed has been flown directly to hospital.
"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," she said.
"He was taken as a priority to hospital escorted by the doctor from London's Air Ambulance. "
My thoughts are with the victim of the horrific stabbing in Leytonstone. Police were quickly on the scene and have arrested a man.
— Sadiq Khan MP (@SadiqKhan) December 5, 2015
One Twitter user has described how a commute to a Christmas dinner with workmates turned quickly to chaos.
"I ignored it and kept walking to get my train, but suddenly what I saw I couldn't believe my eyes," he wrote.
"What I saw was a guy with a knife and a dead guy on the floor. I was so scared I ran for my life.
"After a good 10-15 police came and got the guy and arrested him."
Witness Michael Garcia explained how he could see the slashing victim and the alleged attacker close by.
"I look over and i see a guy huddled on the floor behind the barriers and the guy brandishing the knife," he told ITV News.
IIt was a small blade maybe about three inches long. And then he's screaming 'go on then, run, run' and there's people like wanting to go forward to try and help the guy, but the guy's standing just right next to him, just brandishing the knife."
The incident has now been handed over to the Metropolitan Police's Counter Terrorism Command.
Britain is on its second-highest alert level of "severe," meaning a militant attack is considered highly likely.
This is mainly due to the threat the authorities say is posed by Islamic State militants in Syria and Iraq and their encouragement of supporters to carry out attacks in their homelands.
British MPs approved the bombing of Islamic State targets in Syria on Wednesday.
Britain's air force has since carried out two bombing raids.
Morning news break - December 6