A man was arrested Saturday after allegedly stabbing and seriously injuring a commuter while being heard shouting “this is for Syria” inside a London tube station, Sky News reports.

Police later said the attack, which occurred in the Leytonstone tube station in East London, was being investgated as a “terrorist incident”.

A Twitter user posted video obtained by The Evening Standard of the immediate aftermath of the attack, which shows a pool of blood and the suspect being shot with a tazer gun by police.

Warning: This video contains graphic content

“Police were called at 7:06 p.m. [local time]… the male suspect was reportedly threatening other people with a knife,” a spokesman for Scotland Yard, London’s regional police force, told Sky News. The spokesman said the suspect was taken into minutes later.

The victim, who has yet to be identified, was slashed in the throat area by a machete. Sky News reported that bystanders heard the man shout “this is for Syria” during the attack.

The suspect reportedly began attacking exiting commuters in the Leytonstone station’s main entrance area when the victim was stabbed in their neck.

“We are treating this as a terrorist incident. I would urge the public to remain calm, but alert and vigilant,” Commander Richard Walton, leader of the Metropolitan Police’s Counter Terrorism Command, said.

“The threat from terrorism remains at ‘severe’, which means a terrorist attack is highly likely,” Walton said.

The London Underground’s COO, Steve Griffiths, told the Evening Standard the station is being closed down while police investigate the attack.

Authorities are urging anyone with information or further video of the attack to contact authorities at (+44) 0800 789 321.