(CNN) -- A man stabbed three people at an east London Tube station, and authorities say they are investigating the attack as a terrorist incident.
The assailant, believed to be 29, was subdued with a stun gun and arrested Saturday night, London's Metropolitan Police said.
One man, 56, suffered serious injuries, but they're not considered life-threatening, police said. Two others had minor injuries in the attack.
'This is for Syria'
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.
"We are treating this as a terrorist incident," said Cmdr. Richard Walton, head of the Metropolitan Police Counter Terrorism Command.
"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."
Dijeh said that the suspect had not yet been charged. She would not confirm whether police were seeking anyone else in relation to the attack, saying only that police inquiries were ongoing.
UK faces terrorist threat
The United Kingdom is facing stepped-up threats from ISIS -- especially after British fighter planes began flying sorties last week against ISIS targets in Syria.
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 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.
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).
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.
Videos show arrest
Videos posted to YouTube and on Twitter purport to show the aftermath of Saturday's London stabbing. A large pool of blood is on the ground near the exit gates.
Several show police confronting a man in the ticket area near the exit and yelling, "Drop it! Right now!"
They fire a stun gun at the man, who is wearing a gray top, tan pants and a black hat with ear flaps. But the man continues pacing in front of the officers and even lunges at them.
In one video, a woman can be heard saying, "I just want to get out of here." The scene is loud, with voices echoing throughout the station and police repeatedly shouting at people to get back.
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!"
Hours later, the Leytonstone station was still roped off with blue and white police tape, and police officers were standing guard outside. Investigators wearing protective clothing occasionally went in and out.
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.
CNN's Carol Jordan, Marilia Brocchetto, Kay Guerrero, Nicola Goulding, Andrew Carey and Nick Hunt contributed to this report.
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