A 17-year-old boy is in a critical condition in hospital after being stabbed at a gathering in a West Sussex village.
Sussex Police were called to an address in Warnham, near Horsham, in the early hours of Sunday morning.
“Many young people” were gathered at the location on Marches Road, the force said in a statement.
Photos of the crime scene show police tape surrounding a collection of white tents in the middle of a field, with a portable toilet nearby. It is understood to be the grounds of a nearby large house.
Officers arrested a 16-year-old boy on suspicion of attempted murder. He remains in custody.
Chief Superintendent Jo Banks said: "We are urging witnesses and anyone with information to come forward. We understand that there were many young people in attendance at this address at the time who will know important information to help the investigation.
“In particular we would also ask parents and carers of those who attended to speak with their children and to contact us with information.”
Sussex Police added that it was a “fast-moving and complex” investigation.
Officers will be working in partnership with local schools over the coming days and local people have been told to expect to see an increased police presence in the area.
"We believe this was an isolated incident that does not pose a wider threat to the community," Chief Superintendent Banks added.
Witnesses or anyone with information can report it to Sussex Police on 01273 470101 or online and quote Operation Ketley.