Police are appealing for help in their hunt for a man who raped a 46-year-old woman in a lane at Charing Cross early on Friday.
They particularly want to speak to a man who helped the victim before the police arrived on the scene.
The attack happened between 4am and 4.30am near Renfrew Street, where forensic officers have been scouring the area for clues.
Police are also collating CCTV footage in an effort to gain further evidence.
Detective Inspector Graeme McLachlan said: “We are trying to establish the exact circumstances of what happened and piece together the information we currently have.
“I am appealing to anyone who was in the area who may have heard anything or who may have seen something.
“I am particularly keen to identify a man described as being in his 30s, with short brown hair, who went to the assistance of the woman providing her with water before police arrival.
“If this is you, or you know who it is, please make contact with police as soon as possible.
“I know the incident occurred in the early hours but there may have been passing motorists or people out and about in the area who may have seen something which seemed a little odd or suspicious to them.
“I would appeal to these people to please contact us and pass on any small piece of information.
“You may think it’s not important but it could turn out to be that vital piece of information which leads us to the suspect.”
Anyone with information is asked to call officers at the Divisional Rape Investigation Unit at London Road Police Office via 101. Alternatively, please call Crimestoppers on 0800 555 111 where you can give information anonymously.