A stabbing at a Tube station in east London is being treated as a "terrorist incident", the Met Police has said.
Police were called to reports of people being attacked at Leytonstone around 19:00 GMT on Saturday. The knifeman reportedly shouted "this is for Syria".
A man was arrested after being Tasered by police. One man suffered serious knife injuries while two others received minor injuries.
Metropolitan Police counter-terrorism detectives are investigating the case.
Downing Street said it was monitoring the situation closely.
The Met said the suspect was reportedly threatening other people with a knife.
Fellow passengers reportedly shouted at the man to drop his weapon and one person shouted "You ain't no Muslim bruv," a sentiment which has been picked up on social media by people trying to disown and sideline the attacker.
Another witness told the BBC he saw a man holding a knife, about 3in (7.5cm) long, standing over another man who was lying on the ground, and people running out of the Central Line station.
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."
Another witness, named Khayam, told BBC Radio 5 live people were shouting and throwing things at the arrested man as he was led from the station.
"I saw pedestrians shouting at him, one tried to throw a bottle and the police officers said 'back off' and took him to the police van."
Police said they received a call at 19:06 GMT and the man was arrested at 19:14.
They said the most seriously-hurt victim's injuries were not believed to be life-threatening.
Salim Patel, who works at a shop in the Tube station, said: "People just wanted the attacker to stop it, but then he took the knife out and used it on the victim.
"Everybody left the concourse and the attacker was using the knife on his body then he came out of [the] barrier and the people who were nearby the subway ran off."
He said he had been on the phone at the time to the police and said the attacker left the station and then returned.
Commander Richard Walton, who leads the Met's Counter-Terrorism Command, 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.
"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."
He also urged anyone who took any photos or video of the incident to call that number.
A spokesman for the mayor of London said: "The incident, and the swift and professional response of the officers involved, reminds us of the dangers faced by the police every single day in their efforts to protect Londoners and keep the city safe."
Tube notice board
A spokeswoman for 10 Downing Street said: "There is an ongoing police investigation into the attack in London. We are monitoring the situation closely as further details emerge."
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 the attack "was an act of pure evil".
Iain Duncan Smith, Conservative MP for nearby Chingford and Woodford Green, told the Andrew Marr show: "We cannot let these sort of people, terrorist etc, actually dominate our space." He urged people to carry on going about their business as normal to ensure our freedoms are not "curtailed".
Sadiq Khan, MP for Tooting and Labour's London mayoral candidate, has tweeted a photo of a London Tube notice board that reads: "London Underground say: #YouAintNoMuslimBruv."
"To defeat extremism we must directly challenge their poisonous ideology," he added.