A man was arrested by armed police inside the grounds of the Palace of Westminster on Tuesday.
Officers fired a Taser during the incident by the Carriage Gates entrance to the Houses of Parliament.
The incident is not initially thought to be terror related but police say investigations are continuing.
The Metropolitan Police said the man was arrested "on suspicion of trespassing at a protected site" at 11.55am.
"Enquiries into the circumstances continue," Scotland Yard said.
The entrance was the scene of a terrorist atrocity in March 2017 when Khalid Masood murdered unarmed police officer Keith Palmer.
At the time there was shock at the lack of an armed presence near the gate, and chief coroner Mark Lucraft QC found shortcomings in security at the site meant marksmen did not know where they were supposed to patrol.
Since the attack, security at the site has been beefed up, with a permanent armed presence on the entrance.
The man could be seen being held against a fence inside the Carriage Gates entrance.
Around a dozen police officers were inside the gates with the man.
The man was taken away in a police car shortly after 12.15pm.