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 station just after 19:00 GMT. 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.

A witness told the BBC he saw a man holding a knife, about 3ins (7.5cm) long, standing over another man who was lying on the ground, and people running out of the Central Line station.

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.

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.

"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".

'Thin blade'

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,"  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.

A firefighter enters Westminster tube station in central London, Britain December 4, 2015. The station was closed while the fire brigade investigated reports of smoke coming from a machine room. Photo: Reuters

"I saw a victim come out on a vertical stretcher with ambulance staff and several minutes later police closed off the station and evacuated everyone from the area."

Videos have been posted on the internet, purportedly showing the attacker in the station.

BBC correspondent Helena Lee said: "A number of videos have been posted, unverified, but I have seen a couple of those where we see the suspect near the ticketing hall at Leytonstone Tube station.

"You see this man shouting various things and then he is surrounded by a number of police officers.

"They try to Taser him, they fail a number of times and then they are successful.

"You then see the suspect fall to the ground, drop what appears to be a knife in his hand, that falls to the ground as well and then they arrest him."

'Absolutely shocking'

There has been severe disruption to the Central Line on the Tube since the attack.

No trains were running 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.

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 that the attack "was an act of pure evil".