A 15-year-old girl was subjected to a serious sex attack by a teenage boy at the Epsom Derby, police have disclosed.
The victim has told officers she was attacked in a nearby field by a boy she met in the music and fairground area at the racecourse.
The Queen chose the Epsom Derby on Saturday to start the national Diamond Jubilee celebrations marking her 60-year reign as monarch.
Surrey Police said the sex attack is believed to have happened at around 4pm and was reported to officers just after 6pm the same evening.
Some race-goers have been spoken to by police who appealed for the public's help in tracing the attacker, described as aged between 16 and 17.
Detective Inspector Oana King, of Surrey Police, said: "This was obviously a very traumatic experience for the young girl involved.
"Thankfully this kind of offence is extremely rare and I would like to reassure members of the public that we are doing all we can to identify the perpetrator and bring him to justice.
"We have already spoken to a number of people who were at the Epsom Derby event but are keen to speak to anyone else who may have seen the suspect prior to or after this offence."
Police said they were keen to trace a woman who reported the attack to a member of St John's Ambulance staff after she overheard a conversation about it.
Ms King added: "This woman did the correct thing by reporting her concerns after overhearing a conversation about this incident but unfortunately she left the area before our officers were able to speak to her.
"I would urge this woman to come forward as she may have vital information which could assist the ongoing investigation."
The suspect is white, of skinny but muscular build, with a prominent wide jaw line, short gingery-blond hair and a pale, spotty complexion.
He was wearing a long-sleeved, dark blue top and baggy blue jeans. He was with a boy of a similar age, of a bigger build, with short, flattened, brown hair and tanned skin. He was wearing a white, short-sleeved top and blue baggy jeans.
Anyone with information is asked to call Surrey Police on 101 or Crimestoppers on 0800 555 111.
Peruvian striker Paolo Guerrero shows his dejection after losing 0-1 to Colombia in their Brazil 2014 FIFA World Cup South American qualifier match, at Lima's National Stadium on June 3, 2012. AFP PHOTO/ERNESTO BENAVIDES (Photo credit should read ERNESTO BENAVIDES/AFP/GettyImages)
US pop icon Madonna performs on stage during her first ever concert in the Gulf as part of her MDNA world tour at Abu Dhabi's Yas Island Stadium on June 3, 2012. An estimated 25,000 fans cheered and screamed as the Material Girl finally appeared on stage more than two hours late, wearing a skin-tight black outfit from her 'Girl Gone Wild' album. AFP PHOTO/MARWAN NAAMANI (Photo credit should read MARWAN NAAMANI/AFP/GettyImages)
A man particiates in the 2012 Zombie Fest in Monterrey, Mexico, on June 3, 2012. AFP PHOTO/Julio Cesar AGUILAR (Photo credit should read Julio Cesar Aguilar/AFP/GettyImages)
University students protest againt the Liberal Governament, the bill 78 and the tuition fee increases on June 3nd 2012 in Montreal. The Quebec government stoped the talks with the student groups to end the three-month conflict over the planned hike in tuition. Montreal, Quebec, Canada. AFP PHOTO / ROGERIO BARBOSA (Photo credit should read ROGERIO BARBOSA/AFP/GettyImages)
Nepalese climber Kame Sherpa waves to wellwishers upon his arrival at Tribhuvan Airport in Kathmandu on June 4, 2012, after completing multiple summits of Mount Everest. Sherpa scaled Mount Everest three times between May 19 and 27, a feat he claims is a new world record. AFP PHOTO/Prakash MATHEMA (Photo credit should read PRAKASH MATHEMA/AFP/GettyImages)
Sri Lankan Buddhists hold out flowers and pray during Poya, a full moon religious festival, at the Kelaniya Temple in Kelaniya on June 3, 2012. The predominantly Buddhist Island nation marks every full moon as a key religious holiday. AFP PHOTO/ Ishara S. KODIKARA (Photo credit should read Ishara S.KODIKARA/AFP/GettyImages)
Josh Kerr of Australia surfs during round two during the Volcom Fiji Pro on June 4, 2012 in Tavarua, Fiji. (Photo by Kirstin Scholtz/ASP via Getty Images)
MTV Movie Awards at Gibson Amphitheatre on June 3, 2012 in Universal City, California. (Getty)
Miss Rhode Island USA Olivia Culpo waves to the crowd after winning the 2012 Miss USA pageant at the Planet Hollywood Resort & Casino on June 3, 2012 in Las Vegas, Nevada. (Photo by Isaac Brekken/Getty Images)
Paddle boats participate in Thames Diamond Jubilee River Pageant on June 3, 2012 in London, England. For only the second time in its history the UK celebrates the Diamond Jubilee of a monarch. Her Majesty Queen Elizabeth II celebrates the 60th anniversary of her ascension to the throne. Thousands of well-wishers from around the world have flocked to London to witness the spectacle of the weekend's celebrations. The Queen along with all members of the royal family will participate in a River Pageant with a flotilla of a 1,000 boats accompanying them down The Thames. (Photo by Ian Gavan/Getty Images)