A 31-year-old woman was left unconscious after being grabbed by a man in Edinburgh city centre.
The attack happened as the woman walked in Saunders Street between 2100-2300 BST on Friday.
The alarm was raised after the victim awoke under the nearby McKenzie Bridge and was taken to hospital.
Police said she suffered facial injuries but remembered little of the assault, which they described as "sexually motivated".
A finger tip search of the area is being carried out and Lothian and Borders Police said house-to-house inquiries were also taking place.
The woman, who was heading home, was targeted as she walked along Heriot Row, Gloucester Lane and into India Place.
Det Ch Insp John McKenzie said: "She was not robbed and it would appear to have been a sexually motivated attack."
He added: "She recalls being grabbed as she walked past India Place but then it is a blank until she came round some time later.
"She managed to contact her family who came to look for her and found her under the bridge.
"It does not appear she was robbed although a few of her possessions were found lying nearby."
The woman was taken to Edinburgh Royal Infirmary where she was treated for minor injuries before being released.
An extensive area, including McKenzie Bridge, has been cordoned off while inquiries continue.