A 55-year-old woman was sexually assaulted in daylight in a busy park on Friday.
The woman was walking through Beckenham Place Park in south-east London at 1140 BST when she was seriously assaulted.
She managed to call for help and a passer-by telephoned the police.
Police believe the park would have been full of joggers and people walking their dogs at the time of the attack.
Detectives at Catford CID want anyone who saw anything or who has information to come forward.
A Scotland Yard spokesman said: "They may have seen something they think is insignificant but could help us in our inquiries."