It is understood that the man had been walking along a street prior to the incident and followed the woman into the building.
Detectives have released an E-fit – a computer-generated image- of a man they would like to speak to in connection with the incident and have urged anyone with information to get in touch.
Detective Penelope Rea, the investigating officer from the Public Protection team that covers Croydon, said: “This was an absolutely terrifying ordeal for the victim and she continues to be supported by specialist officers as we continue to work to identify the man responsible and bring him to justice.
“Since it was reported to us, we have carried out extensive enquiries to identify the man responsible. We now really need help from the public to help us identify the man in the E-fit. If you recognise him, please get in touch immediately as we urgently need to speak to him.”
If you have any information that could help the police investigation call 101 or Tweet @MetCC and quote CAD 7368/03DEC.
To remain anonymous, contact the independent charity Crimestoppers on 0800 555 111.