Police are hunting a man who pushed a woman to the ground and sexually assaulted her as she walked on a path near a Weymouth nature reserve.
The victim, aged in her 40s, was attacked in Weymouth Bay Avenue near Lodmoor Country Park and RSPB nature reserve at about 06:30 BST on Sunday.
She was helped by a woman, aged in her 60s, who was walking a black Labrador but she did not have a mobile phone.
She found a man who called 999. Police are trying to trace witnesses.
When the dog walker and man returned to the scene, the woman was being assisted by two runners - one man and one woman.
Det Sgt George Graham, of Weymouth CID, said: "We are determined to identify the offender responsible and I would urge anyone with information to please come forward.
"We are urgently trying to identify the woman who found the victim and the female runner who stopped to help so we can take their accounts as they may have information that will assist us."
The detective added there would be an increased police presence in the area as inquiries continue.