A woman was sexually assaulted on Plumstead Common on Sunday night (June 22) - the area's third sex attack in a week.
Police were called to the common after a woman in her 20s was attacked at around 9.30pm on Sunday.
She had been approached while sitting on a bench in The Slade playground by a man described as oriental, around 5ft 5in tall, of slim build and wearing dark clothes and a hooded top.
Two men and a woman rushed to her aid after hearing her cries for help. The man ran away towards Abbey Wood.
Detectives say the incident is not being linked to a sex assault on Winn's Common last Saturday (June 14) when a jogger had her clothes pulled down.
Nor is it linked to a sexual assault on a 12-year-old girl in Plumstead Gardens on June 15. Four boys aged 12, 13, 14 and 15 have been arrested over that incident.
Officers from Plumstead CID are investigating but there have been no arrests so far over the most recent incident.
Detective Inspector Charmaine Laurencin said: "We are keen to hear from anyone who may have seen or heard anything on Sunday. In particular we are keen to speak to a third member of the public, a man, who assisted the victim but left before police arrived on the scene.
"We would like to reassure members of the public that we have a team of detectives working on this case and exploring all investigative leads."
Anyone with information should call Detective Constable Mike Morgan on 020 8284 9554 or Crimestoppers on 0800 555 111.