A number of people have been injured after a vehicle crashed into security barriers outside the Houses of Parliament, Scotland Yard say.
"We're still trying to piece it together", a spokeswoman said.
The incident happened at 07:37, prompting armed police to swarm the areas.
Officers could be seen surrounding the vehicle before leading a man away in handcuffs.
"Straight away I thought "Oh no, this is another terrorist attack".
A witness, Ewalina Ochab, said: "I think it looked intentional - the vehicle drove at speed and towards the barriers".
"I was walking on the other side [of the road]". No one is allowed in or out.
Westminster Tube Station is also closed nearby after being evacuated. As parliament is in recess for summer, most lawmakers are not using the building.
Dozens of armed police, ambulances and firefighters are on the scene after the emergency happened just after 7.30am on Tuesday, August 14. Some of them plain clothes, some of them in uniform.
The Houses of Parliament are surrounded with security barriers of steel and concrete.
Westminster was the scene of a terror attack last year, when Khalid Masood, a 52-year-old British convert to Islam, drove a auto at pedestrians on a bridge over the River Thames, before fatally stabbing a policeman on guard outside parliament.
The BBC's Callum May, at the scene, said: "Parliament Square is now ringed by blue police tape".