Reporters at the scene said some areas were closed off during this morning's incident
Police in London are "open-minded" over terror being a motive after a man was tasered outside the British parliament at Westminster after allegedly reaching for a knife.
Scotland Yard have not declared a terrorist incident but revealed counter-terror officers will be investigating "given the location, the circumstances and recent tragic events".
The man, in his 30s, was arrested for possession of a knife and remains in custody at a central London police station.
The Met Police said officers on routine duties near to the Carriage Gates entrance to the Palace of Westminster became aware of a man "acting suspiciously" at 11.10am on Friday.
"The officers approached the man in order to speak with him. The man reached for a knife, and police discharged a taser," the Met said.
Scotland Yard said it was "too early to understand the motivation", adding: "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."
Seven armed police officers still at the gates of Parliament, suspect has been driven off in police van pic.twitter.com/W2M7UlKuBL— Jessica Elgot (@jessicaelgot) June 16, 2017
No one was injured.
A bearded man wearing a grey hooded jumper and black trousers was pictured being held against railings with his arms behind his back by a police officer.
The Carriage Gates entrance to parliamentary buildings was closed following the arrest but has since been reopened.
A police riot van, an armed response vehicle and an unmarked police vehicle were seen attending the scene.
A parliamentary spokesman said: "We are aware of the earlier incident outside Carriage Gates. An individual has been arrested.
"The incident has now ended and access to the estate has returned to normal."
On 22 March, PC Keith Palmer was stabbed to death just inside the grounds by terror attacker Khalid Masood, who was gunned down by armed police.
Masood entered the New Palace Yard through Carriage Gates. Following the attack the entrance's gates were replaced by bulkier, less decorative gates with wire meshing.
A review of the complex's security was ordered in the wake of the attack.