A man has been arrested by armed police outside the Palace of Westminster.
The bearded suspect, wearing black trousers and a grey sweatshirt, was led away into a van by officers after being detained for around 30 minutes.
He was held with his arms against his back just yards from the forecourt of the building, where Pc Keith Palmer was stabbed to death by terrorist Khalid Masood in March.
Scotland Yard said the man - in his 30s - was arrested on suspicion of possessing a knife.
There are no reports of injuries, and the incident is not thought to be terror-related.
Security around Westminster was tightened following the attack by Masood, who was shot dead after fatally stabbing Pc Palmer as he guarded Carriage Gate.
A review of security at the perimeter of the parliamentary estate was ordered by Commons Speaker John Bercow following the incident.
The use of armed officers and physical barriers has become more visible, while a permanent barrier has been erected to protect pedestrians on Westminster Bridge, where Masood murdered four people and injured more than 50 others by mounting the pavement in a car.