Detectives believe they have smashed a child trafficking gang preying on runaways as young as 11 for Britain's sex trade.
Thirteen men were arrested in Oxford by police investigating claims 24 vulnerable girls under 16 were groomed for rape.
Officers suspect gang members targeted children in care or living rough and transported them around Britain over a six-year period.
Amid fears more children may be involved, patrols of officers took to the streets of the university city to hand out public awareness leaflets.
Thames Valley Police said nine men - aged between 21 and 37 - remain in custody and are being quizzed over a string of offences including causing the prostitution of females under the age of 18, administrating drugs for the purpose of rape, trafficking, grooming and rape.
Four men, aged 23, 31, 34 and 37, were released on bail until April 19.
Police became aware of the allegations after a number of girls were reported missing.
Detective Superintendent Rob Mason said: "We believe we have uncovered an organised crime group who have been running a business of selling young girls for sex.
"We have also identified a number of customers who we have reason to believe have used this service."
More than 100 officers took part in raids as the force executed 14 warrants across the city as part of an operation named Bullfinch.
Police were not willing to comment on the nationalities of any of the suspects but sources said many of the offences centred in or around the Oxford area.
Many of the alleged victims were said to have been "known" to children's services.
Mason, speaking later at the force's city headquarters, said the inquiry was still at an early stage.
"We consider the girls to be very, very vulnerable and that's why they have been targeted," Mr Mason said.
"The girls have gone missing and when they returned they have made certain disclosures.
"Through children's services and Oxfordshire County Council we have become aware of some of the activities the girls have been getting into when they've gone missing."
Police suspect they have arrested both those behind the running of the network and the men that have paid for sex.
The officer added: "The investigation has been very challenging. The girls are very vulnerable and a number of them do not consider themselves to be victims."
Detectives are keeping an "open mind" as to whether there would be further arrests.
"It is going to be a long investigation," Mason added. "Our priority is the welfare of the girls."
Much of yesterday's activity was centred near St Aldates police station in central Oxford. More than 40 uniformed officers and community support officers were handing out A5 flyers about the operation.
Police chiefs also posted a video on YouTube to reassure community members and urge any potential victims to come forward.
Officers are working closely with children's services, the Child Exploitation and Online Protection Centre and other forces that have been through similar investigations.
Police are also urging anyone who thinks they may have been a victim to contact ChildLine on 0800 1111.