A stabbing at a Tube station in east London is being treated as a "terrorist incident", UK police say.
Police were called to reports of people being attacked at Leytonstone station just after 7pm (local time).
A man was arrested after being tasered by police. One man suffered serious knife injuries while two others received minor injuries.
Detectives from the Metropolitan Police's Counter-Terrorism Command are now investigating the incident.
The suspect was reportedly threatening other people with a knife, The Met said.
A witness said he saw a man holding a knife, about three inches long, standing over another man who was lying on the ground, and people were running out of the Central Line station.
A video purportedly showing police trying to arrest the man was shared on Twitter.
Police said they received a call at 7.06pm and the man was arrested at 7.14pm.
The most seriously-hurt victim's injuries were not believed to be life-threatening.
Commander Richard Walton, who leads the counter-terrorism unit, said: "We are treating this as a terrorist incident. 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.
He urged witnesses to contact police.
Suspect is said to have shouted "This is for Syria". As he was arrested an onlooker said "you aint no Muslim bruv" https://t.co/G0yhRqUJI3— London24 (@london24) December 5, 2015
Witness Michael Garcia, 24, a financial analyst from Leytonstone, said he was walking along an underground passage that runs through the station when he saw people running outside.
"I realised it wasn't a fight but something more sinister," he said.
He then saw "a guy, an adult, lying on the floor with a guy standing next to him brandishing a knife of about three inches... maybe a hobby knife".
"It had a thin blade, but looked fairly long," Mr Garcia said.
"He was screaming 'go on, then, run' to everyone else. He was pacing back and forth next to the guy on the floor. He came up to the barriers."
There has been severe disruption to the Central Line on the Tube since the attack.
No trains will run between Liverpool Street and Woodford and Newbury Park for the rest of the night and there have been lengthy delays on the the rest of the line.
Tickets are being accepted on local buses and Abellio Greater Anglia and TfL Rail trains.
Labour leader Jeremy Corbyn wrote on Twitter: "The stabbing in Leytonstone is absolutely shocking. My thoughts are with the victim and his family."
Tim Farron, leader of the Liberal Democrats, tweeted that the attack "was an act of pure evil".