This is the first picture of a crumpled car outside the Houses of Parliament after hitting a number of pedestrians 'at speed'.
Streets around Parliament Square, Millbank and Victoria Tower Gardens have been cordoned off as dozens of armed police swooped on the scene.
Armed officers could be seen surrounding the car before leading a man away in handcuffs.
A series of ambulances arrived at the scene.
Scotland Yard said: "At 0737hrs today, a car was in collision with barriers outside the Houses of Parliament.
"The male driver of the car was detained by officers at the scene. A number of pedestrians have been injured. Officers remain at the scene.
"We will issue further info when we have it."
The Houses of Parliament are surrounded with security barriers of steel and concrete.
The measures were extended in the wake of the Westminster Bridge attack in March 2017 when Khalid Masood ploughed a car into crowds on Westminster Bridge, killing four people.
Masood abandoned his car then stabbed and killed unarmed Pc Keith Palmer before he was shot by armed police in a courtyard outside Parliament.
More than 10 police vehicles and at least three ambulances remain outside Parliament.
Firearms officers and at least two police dogs are stationed inside Parliament Square.
Officers have cordoned the whole square off and are asking the public and press to move away from the scene.
Eyewitness Ewalina Ochab told the Press Association: "I think it looked intentional - the car drove at speed and towards the barriers."
She said: "I was walking on the other side [of the road]. I heard some noise and someone screamed. I turned around and I saw a silver car driving very fast close to the railings, maybe even on the pavement."
"The person driving did not go out" of the vehicle, she said.
The vehicle did not appear to have a front registration plate when it crashed, she added.
Members of the public and press have been moved back further from the area as police put up a "Terrorism Act cordon", according to an officer at the scene.
The cordon has been extended past the Cenotaph on Whitehall - a five-minute walk from the scene at Old Palace Yard.
Westminster station has been closed for both entry and exit due to the security alert, Transport for London said.
Passengers are still able to change between lines at Westminster.