°®Âþµº

A man is being held in custody on Sunday after three people were stabbed at a London Underground train station in an attack police were treating as a "terrorist incident".

Eyewitnesses claimed that a man screamed "this is for Syria" before attacking three people at Leytonstone tube station and slashing a commuter's throat on Saturday evening.

Police officers were quickly at the scene and used a Taser to subdue the alleged attacker before apprehending him. It took several attempts, according to witnesses.

One witness shouted "You ain't no Muslim, bruv! You're no Muslim, bruv! You ain't no Muslim!" at the suspect as he was pinned down by officers.

The victim suffered serious but non-life-threatening wounds according to reports. At least two others were also hurt in the incident.

Video footage emerged of the attack which shows a pool of blood and the confrontation between the alleged attacker and the police.

"Who is this idiot?" one onlooker said of the suspect, adding: "Yes! Stupid idiot," after the man is Tasered.

Officers shout "Put the knife down!" and "Drop it now!" at the stunned man, who did drop the short-bladed knife as he quivered on the ground.

On his front, the man was handcuffed behind his back while another officer pinned him down with his knee and passers-by were told to get back.

The hashtag was trending on Twitter on Sunday as people reacted to the news.

Terror threat level 'severe'

London's tube stations have been seeing an increased police presence following the Paris attacks and the decision by Britain to get more involved in the military campaign against Islamic State group in Syria.

The Metropolitan Police's counter-terrorism unit is treating the attack as a terrorist incident.

"The threat from terrorism remains at severe, which means that a terrorist attack is highly likely," Commander Richard Walton who leads the counter-terrorism unit said.

Britain's national terror threat level was raised in August 2014 to severe, the second-highest of five levels, meaning an attack is considered highly likely.

"I would urge the public to remain calm, but alert and vigilant," Walton said.

London's transport system was hit in July 2005 by a series of suicide bomb attacks that killed 52 people.

Leytonstone station was open again Sunday, with a police officer standing guard outside.

The station is decorated with mosaics depicting film scenes by director Alfred Hitchcock, who hails from Leytonstone, as does football icon David Beckham.

A spokeswoman for Prime Minister David Cameron's Downing Street office said: "There is an ongoing police investigation into the attack in London. We are monitoring the situation closely as further details emerge."