A man was reportedly tasered by armed police and held inside the grounds of the Houses of Parliament today.
The Met Police told City A.M. it is trying to work out what happened, but said that no one has been injured.
Police said it was too early to say if it was a terror-related incident, and they have not yet found a weapon on the scene.
"A man was detained and arrested by Carriage Gates inside the Palace of Westminster on suspicion of trespassing at a protected site, at around 11.55hrs on Tuesday, 11 December," the Met said.
"A taser was deployed. Enquiries into the circumstances continue."
A picture from the scene shows a man in red shoes, jeans and a green jacket being held down by three police officers while a third one points a gun at him.
Reports suggest the intruder has now been taken away by police.
Intruder has now been taken away in a police van - he made it a few metres into the parliamentary estate but was brought to the ground pretty quickly - not clear exactly what he was trying to do or what happened— Laura Kuenssberg (@bbclaurak) December 11, 2018
The House of Commons said the situation is now over.