British police say they are investigating an unprovoked knife attack in an east London Tube station Saturday, which left a man with serious injuries, as a terrorist incident.
The assailant, 29, was subdued with a stun gun and arrested on suspicion of attempted murder at 7:14 p.m. Saturday in the suburb of Leytonstone, London's Metropolitan Police said.
Police had been dispatched to the station after receiving emergency calls reporting that a man had stabbed people and was threatening others, police said. The man threatened officers as they tried to reason with him, a statement said.
Police are looking into reports that the suspect yelled, "This is for Syria," during the attack, said Chioma Dijeh, a spokeswoman for the Metropolitan Police.
One man, 56, suffered serious knife injuries in the attack and was taken to an east London hospital, where he remains in a stable condition. His injuries are not considered life-threatening, police said.
A second man received a minor injury that did not require medical treatment, while a woman was also threatened by the suspect, police said.
Authorities initially said one person was seriously injured and two others lightly injured.
Cmdr. Richard Walton, head of the Metropolitan Police's Counter Terrorism Command, said police were investigating the stabbing as an act of terror.
"As a result of information received at the time from people who were at the scene, and subsequent investigations carried out by the Counter Terrorism Command ... I am treating this as a terrorist incident," he said in a statement.
He urged the public to "remain calm but alert and vigilant" in relation to any potential threat. "The threat from terrorism remains at severe, which means that a terrorist attack is highly likely."
Dijeh, the Metropolitan Police spokeswoman, said no charges have been filed. She could not confirm whether police were seeking anyone else in relation to the attack, saying only that police inquiries were ongoing.
Detectives were searching a residential address in east London on Sunday afternoon as part of the investigation, police said.
Detective Superintendent Jacqueline Sebire of the local Waltham Forest police force praised the attending officers for "not only their bravery in confronting the man who was in possession of a knife, but also their quick response in providing first aid to the victim."
She appealed for anyone in the area at the time who may have seen something suspicious to call an anti-terror hotline.
Videos show arrest
In the wake of the stabbing, footage circulated widely on social media showing scenes from the bloody attack and its chaotic aftermath.
They show police confronting a man in the ticket area of the station, who is brandishing an object in his hand. The police yell: "Drop it! Right now!"
They fire a stun gun at the man, who continues pacing in front of the officers, then lunges at them, causing bystanders to back off in fear.
There is more shouting before a pop is heard, apparently from the stun gun, and the man falls to the ground. Someone watching shouts, "Yes! Stupid idiot!"
Two police officers roll the man onto his stomach and handcuff him.
A man in the crowd shouts, "You ain't no Muslim, bruv! You're no Muslim, bruv! You ain't no Muslim!"
The phrase was embraced by social media users as a statement of defiance toward the attacker and his perceived motivations.
Sadiq Khan, a London member of Parliament and candidate for the city's forthcoming mayoral election, tweeted a picture of what appeared to be a London Underground information board: "London Underground say #youaintnomuslimbruv."
Jeremy Corbyn, the leader of the opposition Labour Party, tweeted that the attack was "absolutely shocking," while Tim Farron, leader of the Liberal Democrats, tweeted that it was "an act of pure evil by a psychopath on innocent people."
UK faces terror threat
The United Kingdom is facing stepped-up threats from ISIS -- especially since British fighter planes began flying sorties last week against ISIS targets in Syria. The strikes were launched after British lawmakers voted to extend their existing military efforts against the Islamist terror group in Iraq into Syria in the wake of the Paris attacks.
Intelligence obtained by European security agencies indicates ISIS is aiming to attack the UK as a follow-up to its attacks in Paris last month, a senior European counterterrorism official has told CNN.
Speaking in Parliament last week, British Prime Minister David Cameron referred to seven ISIS plots against the country that had been foiled in the past year.
The threat to the UK from terrorism has been rated as "severe" since August 2014; since the threat level was made public, in 2006, it has been raised to critical on two occasions.
Attack halts Tube travel
The Tube, also called the London Underground, is the city's subway system. It has 270 stations on 11 lines that stretch a total of about 400 kilometers (250 miles).
Leytonstone is on the Central line, which runs roughly west to east through central London and into the northeastern suburbs.
Transport for London, which runs the Underground, shut down a large part of the eastern Central line after the stabbing, but the service was running as usual Sunday. Leytonstone station reopened early Sunday morning, police said.
In 2005, suicide bombers attacked three Underground trains and a double-decker bus in a coordinated strike that left 52 people dead and more than 770 wounded. A British al Qaeda operative planned the bombings, according to internal al Qaeda documents that surfaced in 2012.