London’s Metropolitan Police said a number of pedestrians were injured in the incident but at this stage none of them are believed to be in a life-threatening condition.
Footage on social media showed a handcuffed man being led away by heavily armed police.
Eyewitnesses said the driver of the Ford Fiesta drove from one side of the road to the other, striking a group of cyclists before hitting the barricades.
“Someone [was] driving towards parliament at speed, it wasn’t normal, it’s not a minor, like, accident, it’s not like a road traffic incident,” witness Jason Williams told UK’s Sky News.
“This is something in my opinion planned and very serious.”
Two people were treated at the scene for injuries that were not believed to be serious, London Ambulance service said. Media reports said a cyclist was among the injured.
Television images showed police pointing their guns at a vehicle which appeared to have entered a lane used for access to the Parliament building before it stopped at a security barrier.