A young woman was sexually assaulted after she was offered a lift home from V Festival, police said.
The victim, in her 20s, was attacked by a man driving a dark Mondeo on Greenbury Way in Chelmsford between 9.30pm and 10.30pm on Saturday.
Essex Police said the woman entered the car believing it was a taxi after she was offered a lift.
She was driven a short distance before she was sexually assaulted and raised the alarm when she managed to leave the car to run away.
Police are investigating the incident and are 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."
V Festival is taking place this weekend in Hylands Park, Chelmsford and Weston Park, Staffordshire.
Anyone with information should call police on 101 or contact Crimestoppers anonymously on 0800 555 111
