Heroic members of the public helped tackle a suspected knifeman in Leicester Square in a horror incident that left four people injured.
Police rushed to the scene just after 1am on Sunday to reports of a man with a knife threatening members of the public.
Footage posted online shows a group of apparent passers-by pinning the alleged attacker to the floor.
In the video the man, who was wearing a black coat, trousers and a blue rucksack, holds up what appears to be a knife as he is held to the ground.
He then orders everyone to "get away" before standing up.
The suspected knifeman is then dragged to the floor again by a hero security guard, before other security arrive and he is detained.
Blood can be seen covering the alleged attacker's hands.
The footage then shows the security guard - who appears to have suffered a injury to his face in the incident - apparently finding a large knife on the ground, picking it up and taking it away.
The actions of the security guard were praised online, with people naming him as "Waqar".
"He put himself in harms way to help and save others. He's a family man as well. I hope he gets rewarded for his heroic efforts," one person wrote next to the video on Instagram.
A spokesperson for the Metropolitan Police said: “We were called at 01:08hrs on Sunday, 21 September to Leicester Square, Westminster to reports of a man with a knife threatening members of the public.
“Officers attended and found four men, aged in their 20s and 30s, with injuries. They are not believed to be life-threatening or life-changing.
“A 50 year-old-man was arrested at the scene on suspicion of grievous bodily harm, possession of an offensive weapon, grievous bodily harm, affray and possession of a Class B drug. He has since been detained under the Mental Health Act.
“Anyone with information is asked to contact police on 101 or online, quoting CAD 445/21SEP. Or you can contact Crimestoppers anonymously on 0800 555 111.”
