Police were called at 6:55pm on Friday (15th) to reports of a collision on Fishergate Hill.
It involved a Scania double decker bus and a pedestrian.
The bus was travelling uphill in a bus lane towards Preston at the time.
The pedestrian, a 32 year old woman, suffered serious injuries and was taken to Royal Preston Hospital where her condition is described as critical.
Nobody on the bus was injured.
Police are appealing for witnesses and are making a direct appeal to a male pedestrian who stopped at the scene.
It is believed this person commented to someone else that they had witnessed the incident but unfortunately they left before police arrived at the scene.
Sgt Phil Baxendale of Lancashire Police’s Tactical Operations Division, said: “Our thoughts first and foremost are with the injured woman at this incredibly difficult time.
"We need to establish what happened and would urge anyone who can help to make contact as soon as possible.
“If you are the male described above, please come forward. You could hold vital information.
“We are conducting thorough CCTV enquiries of our own, however if you have a business in the area and your cameras may have captured the incident, please let us know. Likewise, if you were travelling through the city centre around this time and have a dash cam, please check it for any relevant footage.
“Finally I would like to ask that anyone who was travelling on the X2 bus into Preston at the time of the collision who hasn’t already spoken to police, to call us if they have any information at all.
“We would like to thank the Preston public for their patience in this matter. Due to the scene closure, we understand that a number of people have had to walk a considerable extra distance to get to their destination and we appreciate their understanding and co-operation with this.”
If you have any information that could assist our investigation, call 101 or email [email protected] and quote log number 1309 of 15th November.