A man was injured in a jail in front of the Home Office in London, police said.
The city police were summoned shortly after 1:00 pm to the area in front of the building on Marsham Street, after a man had been reported with a knife.
On his arrival, a man with knife injuries was found and taken to the hospital for treatment.
It is not believed that his injuries are life threatening.
The Met said that investigations are ongoing to establish the circumstances of the incident and that terrorism as a motive has not been ruled out.
A man was arrested and detained on suspicion of grievous bodily harm.
Eyewitness Gareth Milner, who had lunch near the Home Office at the time of the incident, said he had seen "several armed police officers arrive and walk in the building.
He added, "Some time later, a gentleman with a rather bloody face and a bandage pressed into his face was escorted by the Home Office of paramedics.
"He was calm and walked on his feet without being physically supported."
Another eyewitness, a barista working opposite the Home Office, said she saw a man "full of blood" coming out of the building.
"He only had pants on, nothing on top," she said. "He was full of blood and was accompanied by police and paramedics."
An online photo showed a bloody, lousy man helped by a policeman and a paramedic.
An office clerk told the Interior Ministry that they believed they saw the offender "quietly" gone after the incident.
They said, "He was standing outside the Home Office and then he put something in his bag, but I did not see a knife or anything.
"He cleared away a bit and then went down the street quite calmly, as if nothing had happened.
"Then people were asked to go to the Home Office, they were brought in."
The London Ambulance Service said in a brief statement that the incident had been answered in addition to the Met.
It added, "We dispatched a dispatcher, an ambulance driver, a motorcycle paramedic, and an ambulance crew.
"We treated a local man and took him to a large trauma center."
In a tweet, Interior Minister Priti Patel said her thoughts were with the victim and his family after the "terrible, unprovoked knife attack".
A spokesman for the Home Office said in a statement, "This is a deeply worrying incident, and our thoughts are with him and his family.
Since it is an ongoing investigation, it would be inappropriate to make further comments. "
Currently there is a barrier around the scene, two police cars and several more are nearby.