A man has been arrested on suspicion of possessing a knife outside the Houses of Parliament in Westminster, with the Metropolitan Police stating they are keeping an "open mind" as to whether terrorism was a motive.
Eye-witnesses described seeing a man "tasered" by armed police outside the House of Commons after shouts of "knife, knife, knife".
Armed police reportedly swooped on the man and discharged a taser before detaining him.
Witnesses said the gates to the Palace of Westminster were immediately closed "as a precaution".
A spokesperson for the Metropolitan Police said: "At 11.10hrs on Friday, 16 June, officers on routine duties near to the Carriage Gates entrance to the Palace of Westminster became aware of a man acting suspiciously.
"The officers approached the man in order to speak with him. The man reached for a knife, and police discharged a taser. The man - aged in his 30s - was arrested for possession of a knife.
"At this time it is too early to understand the motivation so we have not declared this a terrorist incident. However given the location, the circumstances and recent tragic events, the MPS Counter Terrorism Command will be investigating this incident.
"We remain open minded as to whether terrorism was a motive."
The spokesman added: "Once again we commend the actions of our armed and un-armed officers based in Westminster who are keeping the public safe."
A Parliamentary spokesperson said: "We're aware of an incident that's taken place outside the Palace of Westminster, which is being dealt with by the Metropolitan Police."
The arrest comes two months after a terrorist murdered five people, including a police officer, PC Keith Palmer, outside the Houses of Parliament.
Khalid Masood drove a car onto the pavement on Westminster Bridge, killing four pedestrians, before crashing into the gates of the Palace of Westminster and stabbing PC Palmer.