Three people were killed and others were left seriously injured when a coach thought to be returning from a music festival overturned after hitting a tree on an embankment.
Emergency services were working at the scene after the coach, which was reportedly returning to Merseyside following the Bestival event on the Isle of Wight, left the A3 in Surrey at 11.50pm on Monday night between the Hindhead tunnel and the exit for Thursley.
No other vehicles were believed to have been involved.
Three people aboard the full 51-seat coach were declared dead at the scene and a fourth passenger was airlifted to Southampton General Hospital with life-changing injuries, Surrey Police said.
Other passengers, some of whom have serious injuries, were taken to the Royal Surrey County Hospital in Guildford, St Peter's Hospital in Chertsey, St George's Hospital in Tooting, King's College Hospital in London, Frimley Park Hospital and Queen Alexandra Hospital in Portsmouth.
The road was closed in both directions initially and Surrey Police, Surrey Fire and Rescue and South East Coast Ambulance Service all attended the scene, along with collision investigators. The southbound carriageway later reopened.
The coach is understood to have been operated by bus company MerseyPride and Surrey Police are working with their counterparts at Merseyside Police, as well as with the company, to identify the passengers and notify relatives. Other passengers were taken to a rest centre which was set up nearby by Waverley Council.
Surrey Police Assistant Chief Constable Jerry Kirkby said: "This has been declared a major incident and we are working with our colleagues from the other emergency services and other partner agencies to provide a co-ordinated response in line with agreed protocols.
"Our thoughts remain with the relatives of the dead and the injured at this time and we are working very hard to notify their next of kin."
A casualty bureau number has been set up and people concerned about passengers on board the coach should call 0800 056 0154.