A 25-year-old woman was raped in Burgess Park in Camberwell in the early hours of Sunday morning.
A 31-year-old man was arrested on suspicion of rape and robbery later on Sunday.
He remains in custody.
Police were called to the park off St Georges Way in SE15 just after 5.20am on Sunday.
A 25-year-old woman was taken to hospital to be assessed and she is being supported by specialist police officers.
A crime scene is in place.
Anyone with information should call police on 101 with the reference 1448/25SEP.
A Metropolitan Police spokesperson said: "At 05:23hrs on Sunday, 25 September police were called to Burgess Park off St Georges Way, SE15 following reports that a woman had been sexually assaulted.
"Officers attended and a 25-year-old woman was taken to hospital for assessment. She is being supported by specialist officers.
"Later the same day, a 31-year-old man was arrested on suspicion of rape and robbery.
"He remains in custody.
"Anyone with information should call police on 101 with the reference 1448/25SEP.
"Information can also be provided to Crimestoppers, anonymously, on 0800 555 111."
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