The area is in lockdown after armed police swarmed the scene on Tuesday morning.
Images posted to social media showed a suspect surrounded by officers being led away from the vehicle in handcuffs.
Streets around Parliament Square, Millbank and Victoria Tower Gardens have been cordoned off while police, ambulances and firefighters arrived at the scene.
The Metropolitan Police said on Twitter: "At 7.37am 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."