A government employee has been stabbed outside the Home Office in central London.
The victim, who is in his 60s, suffered non-life threatening injuries on Marsham Street, near the Houses of Parliament, just after 13:00 BST.
A 29-year-old man was arrested on suspicion of causing grievous bodily harm and possession of CS spray.
Police said it was not being treated as terror-related, but they were “keeping an open mind” about possible motives.
The wounded man went into the building to seek assistance, an eyewitness said. The building was put into lockdown.
The Ministry of Housing, Communities & Local Government said the man was a government employee.
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.”
He said security will be reviewed across the department, which shares the building with the Home Office.
Home Secretary Priti Patel sent out a tweet saying her thoughts were with the victim and his family following the “unprovoked knife attack”.
Eyewitness Gareth Milner said he was outside the Home Office when “a number of armed police officers arrived on scene and entered the building”.
“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,” said Mr Milner.
He said the man was “walking on his own two feet”.
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.”