A man has been detained at the scene and streets around Parliament Square, Millbank and Victoria Tower Gardens have been cordoned off.
Scotland Yard said in a tweet: "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."
Armed officers could be seen surrounding the car before leading a man away in handcuffs.
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.