A 29-year-old man who was arrested by armed police near an entrance to the Houses of Parliament has been detained under the Mental Health Act.
About a dozen officers swarmed around the suspect as he was held against a fence inside the Carriage Gates on Tuesday.
The man was Tasered after he ran towards officers and failed to stop, the Metropolitan Police said.
He was arrested on suspicion of trespassing at a protected site and taken to a police station for questioning. He has since been detained following a mental health assessment.
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 at the entrance.