Armed police detained a man on the grounds of the British parliament on Tuesday, at almost exactly the same spot where an attack occurred last year, an AFP photographer saw.
London's Metropolitan Police did not immediately respond to a request for information about the incident, while parliamentary authorities said police were handling it.
The BBC cited police saying the incident was not terror-related.
In March last year, Khalid Masood, a British Muslim convert, rammed a car into pedestrians on Westminster Bridge before fatally stabbing a policeman on guard outside parliament.
The attack, which left five people dead and around 50 injured, ended only when police shot the 52-year-old Masood dead.