An attacker grabbed a woman and put his hand over her mouth before sexually assaulting her.
The victim was walking along Drop Redoubt Road in Dover in the middle of the day when she was targeted.
He initially tried to make conversation with her, but when she tried to get away, he put his hand over her mouth before launching the attack.
Police are now searching for a dog walker who saw what was happening and shouted at the suspect, prompting him to run off.
Investigating officer Detective Constable Stuart Tracey said: "We would like the dog walker to get in touch as he may be able to help with our investigation.
"The victim described him as a white man, 60-years-old with thinning grey hair and wearing a white shirt. He was walking a brown Labrador.
"If there were any other witnesses in the area at the time then we would also urge them to get in touch."
The attack happened at about 1pm last Wednesday, August 3.
The suspect is described as a white man, wearing a blue and white horizontal striped polo shirt with the collar turned up, blue denim three-quarter length jeans.
He was about 5ft 5ins in height, proportionate build with mousey, short untidy hair and was clean shaven.
Anyone with information is asked to call police on 01843 222289 quoting reference ZY/026691/16, or Crimestoppers anonymously on 0800 555111.