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 also 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."