Police are appealing for any witnesses or information concerning a sexual assault in Sumner Buildings on Bankside's Sumner Street on Thursday 29 September.
It occurred around 9.15pm in the stairwell of a block of flats accessed from Sumner Street.
A 25 year old woman was using the stairs to one of the blocks when she was suddenly approached by a man on the stairs. He tried to speak to her but when she ignored him, he grabbed hold of her. He pushed her down on to the stairs and assaulted her.
The victim fought back. Fortunately, a resident of the flats was climbing the stairs and interrupted the attack.
The suspect walked back down the stairs and the resident alerted police.
The victim sustained injuries to her neck and was taken to St Thomas' Hospital and treated for bruising and cuts.
The suspect is described as a black man aged between 25 and 35, he is very dark skinned and speaks with an African accent. He is about 5'10 tall and of medium build.
The incident is being investigated by the Southwark Sapphire Unit, based at Walworth. DC Amelia Carter said: "There is no doubt the attack would have been a great deal more serious if the resident had not come along when he did. Although she was so shocked by the suddenness of this attack, the victim did try to defend herself.
"It's possible the man was marked during the course of the incident. Do you know anyone of this description who had black eye, cuts or bruises to his face after Thursday 29 September?"
Anyone with any information should contact DC Carter on 0207 232 6182 with any information, or Crimestoppers on 0800 555 111 if you wish to remain anonymous.