The suspect then took valuables from the victim’s bag before he fleeing the scene on foot.
The woman was taken to a specialist care unit but had been left “very traumatised” by the attack, police said.
DC Sarah Hunter, of the Met’s sexual offences unit, said: “This was a horrific attack on a woman who had stopped to help a stranger when he asked her for some change.
“My team are keen to hear from anyone who has information that can assist the investigation.”
She added: “This was a callous attack which has left the victim very traumatised.”
The attacker is described as a dark-skinned black man of medium build, between 5ft 4in and 5ft 6in tall, and aged between 30 and 40.