Police were called to the Home Office on Thursday after a man was stabbed outside the government building in Westminster.
A 29-year-old man has been arrested on suspicion of grievous bodily harm after the assault on Marsham Street in Westminster.
The victim, aged in his 60s, went inside the Home Office for help and was later pictured being escorted out of the building covered in blood.
A man, who an eyewitness said could have been the perpetrator, was apparently seen putting something into his bag and then walking "calmly" down the road after the incident.
The arrested man was detained in nearby Smith Square and is also suspected of being in possession of CS spray, the Met said.
Police were later pictured inspecting a rucksack and mobile phone which were reportedly dropped by the suspect.
The force confirmed that an animal carrier box also left at the scene contained a ferret.
The Metropolitan Police said they were called shortly after 1pm on Thursday to a report of a man with a knife on the street.
They said inquiries are ongoing into the exact circumstances of the incident.
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Gareth Milner, 34, who works in a nearby building, told Yahoo News: “There was a lot of commotion.
“I was some distance away but there was a sudden rush of people towards the entrance of the Home Office.
“A short while later it felt like half of London’s armed police turned up.
Mr Milner said there were several police cars on the scene within minutes and a number of ambulances.
Lots of armed police and a cordon going up outside the Home Office pic.twitter.com/aj0RqDnPsm— Gareth Milner (@sw1a0aa) August 15, 2019
“After a while, I saw a gentleman who was being escorted out by paramedics walking out under his own steam. It looked almost like he’d been punched in the face.
“There was quite a few staff from the building around the entrance. They were cordoning off the area and officers told us to get clear.”
“There was a few people who didn’t know what was going on and were really shocked. The cordon went up as the man came through and police vehicles were rushing past.”
Shortly after the incident the Metropolitan Police said in a statement: “Police were called at 13:06hrs on Thursday, 15 August to Marsham Street, SW1 following a report of a man with a knife at the location.
“Officers attended to find one man with knife injuries.
“London Ambulance Service attended the address where they are with the injured party.
All my thoughts are with the victim and their family following horrific unprovoked knife attack on Marsham Street.— Priti Patel MP (@patel4witham) August 15, 2019
“One man (no further details) 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.”
When contacted by Yahoo News the Met said they were “keeping an open mind” as to whether the incident was terror-related.
A spokeswoman for the Home Office described the stabbing as "deeply concerning".
She 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."
In July this year the Office for National Statistics revealed there had been 43,516 offences involving knives or sharp objects between January and March 2019 - the highest since comparable records began in 2011.