A woman was sexually assaulted as she made her way home from V Festival, police have said.
The victim, in her 20s, was attacked by a man driving a dark Mondeo on Greenbury Way in Chelmsford, Essex, between 9.30pm and 10.30pm on Saturday.
The woman got into the vehicle after being offered a lift, believing it was a taxi, Essex Police said.
The man drove a short distance before sexually assaulting the woman, who then managed to get out of the car and run away to raise the alarm.
Police are investigating the incident and providing support to the victim.
Detective Sergeant Danny Cooper, of the sexual offences investigation team, said: "I would advise people to be vigilant when travelling alone and to ensure they only use taxis from companies they have contacted themselves or they have found at authorised taxi ranks.
"Any authorised taxi service will display a plate on the rear of the vehicle and a licence containing a photograph of the driver inside the vehicle. If either of these are not on display, then play it safe and don't get in."
Anyone with information should call police on 101 or contact Crimestoppers anonymously on 0800 555 111.