The woman, who is said to have face, head and arm injuries, is believed to have fallen from the bridge in Blackpool Road onto Riversway below.
She was found at around 3.45 and police immediately closed off the eastbound carriageway of Riversway and diverted traffic.
An hour after the incident a police spokesman said: “A woman was found with some serious injuries where Riversway passes under Blackpool Road.
“It looks like she has either fallen from the bridge or been hit by a vehicle. At this stage the suggestion is she has fallen, although we are not sure yet.
“She is OK, she is conscious and she has gone to the Royal Preston Hospital for treatment. She has facial and head injuries and we think a broken arm too.
“The incident has meant the road has had to be closed to traffic and there have been some delays. But we hope to have it open again soon.”