Police are directing traffic away from the scene near the Elephant and Castle pub and investigators are at the scene.
The collision, which involved a white Mercedes van and a female pedestrian, happened shortly before midday. The pedestrian was taken by ambulance to hospital.
Due to the injured woman’s injuries being more serious than initially thought, the decision was taken at 2.20pm to fully close Westgate between Mulberry Way and Quebec Street to allow enquiries to be made at the scene.
A 25-year-old man has been arrested on suspicion of causing injury by dangerous driving and is currently in custody.
Motorists are being advised to avoid the area.
Anyone who witnessed this collision is asked to call 101, quoting log 643 of 05/03.