Armed police, ambulances and firefighters responded to an incident in central London, which happened shortly after 07:30 BST.
Officers were seen surrounding the silver car, which had crashed into a barrier outside Parliament, before a man was arrested.
Streets around Millbank and Parliament Square were cordoned off for several hours after the crash.
A vehicle swerved into cyclists and pedestrians, injuring three people.
Armed police were seen on Victoria Embankment in Westminster.
One women is being treated in hospital for serious but non life-threatening injuries.
Scotland Yard have said they are treating the crash as a terrorist incident.
Forensic officers examined the vehicle at the scene.