A man has been arrested by armed police officers after it is believed he broke into the grounds of Parliament.
An eyewitness told the BBC the man jumped over railings before being Tasered by police.
The man was held within the parliamentary courtyard but has now been taken away in a police van.
Police say he has been arrested on suspicion of trespassing at a protected site and is now being investigated by the Met's counter terrorism command.
The Metropolitan Police said the man ran towards officers and a Taser was used when he did not comply with their requests to stop.
He is to undergo a mental health assessment, police said.
Parliamentary authorities say the incident, which happened just before midday on Tuesday, is contained.
Christopher Hope, a reporter for the Daily Telegraph, told BBC News: "I heard a scream and then looked out the window, and then about half a dozen police officers who appeared not to be armed were running towards a man.
"And then it appears the man was Tasered.
"It appears to be contained now. There are armed police patrolling downstairs by the Christmas tree."
BBC political editor Laura Kuenssberg, who also witnessed the disturbance, says she saw the man on the ground, arguing and shouting at police.
Police gathered up his belongings, including what looked like a passport, before bundling him into the back of a police van, she added.
The incident, which is still in the very early stages of being investigated, happened in the same area inside Parliament's grounds as last year's terror attack.
PC Keith Palmer, who was guarding the gates, was stabbed and killed by Khalid Masood, moments after he had knocked down pedestrians indiscriminately on Westminster Bridge, killing four people. Masood was shot dead by armed officers.