Just after half 7 this morning after a car crashed into security barriers outside the house of parliament in London injuring several pedesstrians.
Reports say cyclists were hit but police do not believe anybody is in a life-threatening condition.
London Ambulance Service said two people were treated at the scene and have been taken to hospital.
Scotland Yard cannot say if the incident is related to terrorism at this stage but a number of eyewitnesses have said the car appeared to deliberately hit members of the public.
Officers were seen surrounding the vehicle before a man was arrested.
The Met Police said:
"While we are keeping an open mind, the Met's Counter-Terrorism Command is leading the investigation into the Westminster incident."