A 17-year-old boy has been taken to hospital with serious injuries after he was stabbed in a town centre.
Hertfordshire Police said it was called by East of England Ambulance Service to Water Lane, Watford, at about 20:15 BST, on Saturday, to reports of an assault.
Road closures remain in place around Lower High Street while officers conduct inquiries. There is no access for vehicles and pedestrians.
The force confirmed there had been no arrests.