Police in Leeds are searching for a gang of men who carried out a "brutal and vicious" attack on a couple as they walked through a park.
The woman was restrained and subjected to a sustained sex attack in a secluded wood, said West Yorkshire Police.
A spokesman said the couple - in their 20s - were walking in the Chapel Allerton park in Leeds on Saturday.
Detective Superintendent Chris Thompson said: "The victims have suffered an horrendous ordeal."
The spokesman said the attack began as the couple were walking through the park, when the woman was verbally abused by the men.
Her boyfriend was also verbally abused and his girlfriend fled into a nearby wooded area.
After a short while she became aware of three of the group approaching her, police said.
She was then physically restrained and subjected to a series of serious sexual assaults.
Her boyfriend was also physically attacked. Police said he managed to recover and the suspects made off when he went looking for his girlfriend.
The woman was left extremely distressed by the attack and is being helped by specially trained police officers.
Her boyfriend suffered injuries and has had treatment.
Det Supt Thompson said: "This was a brutal and vicious assault against an innocent, vulnerable young couple.
"It is imperative that we catch their attackers as quickly as possible."
He appealed to anyone who may have seen anything suspicious in the park between 0700 and 0800 BST on Saturday to come forward.
"Likewise, anyone with information about who they think may be responsible for this sickening attack should ring us immediately," he added.