Video report by ITV News Correspondent Martha Fairlie
A government worker in his 60s has been stabbed outside the Home Office building near Westminster.
The injured man went inside the Home Office, which was put on lock-down, to get help following the assault at around 1pm on Thursday.
A 29-year-old man, who was apparently carrying a ferret in a pet carrier, was arrested on suspicion of grievous bodily harm and possession of CS gas near to the scene.
Police confirmed the suspect has been taken into custody at a central London police station.
In their initial statement, police said the victim suffered life-threatening injuries, but this was later changed to non-life threatening.
It later emerged the victim works for the Ministry of Housing, Communities and Local Government (MHCLG), which is based on the same street as the Home Office.
Communities Secretary Robert Jenrick said: "I am deeply shocked by this horrific attack on a colleague today.
"My thoughts, and those of all my staff, are with him and his family. We are ready to provide as much support as we can and we all wish him a speedy recovery.
"We will be reviewing security across the MHCLG estate. All our staff must feel safe as they go about their important work."
Photographers at the scene captured the moment a man with a bloodied face was escorted out of the building on Marsham Street.
A barista working opposite the Home Office said she saw a man "full of blood" walking out the government building.
She said the victim appeared to be in his 50s and was shirtless as he was helped out by paramedics.
The eyewitness said: "He had just trousers, nothing on top... He was full of blood. He was escorted by police and paramedics...
"I saw just the police when the police came. He was coming out from the building - the officers come in and take him out."
Gareth Milner said he was eating his lunch on the wall outside the Home Office when "a number of armed police officers arrived on scene and entered the building".
Mr Milner added: "Some time later a gentleman with quite a bloodied face and with dressing pressed into his face was escorted out of the Home Office by paramedics.
"He was quiet and walking on his own feet, not being physically supported.
An office worker, who asked to remain anonymous, said she saw a man - thought to be the perpetrator - walking calmly down the road after the stabbing.
She said: "He was standing over there outside the Home Office and then he was putting something away in his bag, but I didn't see a knife or anything.
"He was putting something away and then he just calmly just walked off down the road as if nothing had happened.
"Then people were being asked to go into the Home Office, they were being ushered in."
Eyewitness Guy Ellis, 54, described seeing a man remove a "long" knife from his bag before the incident.
Mr Ellis said he saw a man "take a knife out his bag, and just wander up the path" before he stopped in front of two girls. The man then crossed the road and blended in with the crowd.
He described the man as around 6ft tall, slim with long, dark hair in a ponytail and was dressed "scruffily".
ITV News Political Correspondent Daniel Hewitt said police are inspecting a bag and other items on the corner of Smith Square, which is around the corner from the Home Office.
Home Secretary Priti Patel tweeted: "All my thoughts are with the victim and their family following horrific unprovoked knife attack on Marsham Street."
In a statement, the Metropolitan Police said: "Officers attended to find one man with knife injuries.
"London Ambulance Service attended the address where they are with the injured party.
"The injuries are currently being treated as life threatening."
"One man has been arrested on suspicion of grievous bodily harm and taken to a police station.
"Enquiries are ongoing into the exact circumstances of the incident."
A London Ambulance Service spokesperson said: "We were called at 1:11pm today (15 August) to Great Peter Street, SW1, to reports of an incident.
"We dispatched an incident response officer, a medic in a response car, a motorcycle paramedic and an ambulance crew.
"We treated a man at the scene and took him to a major trauma centre."
A Home Office spokesperson said: "The Metropolitan Police is investigating a knife attack which took place outside the Home Office this afternoon. The victim is receiving medical care.
"This is a deeply concerning incident and our thoughts are with him and his family.
“As this is an ongoing investigation it would be inappropriate to comment further.”