Thirteen people were taken to hospital, including two with life-threatening injuries, after two buses crashed in a city centre.
A major incident was declared by police following the collision in Brighton, East Sussex, at around 9.20am.#
Eyewitnesses said one of the double deckers careered into the back of the other as it was stationary in North Street.
Investigators are exploring theories including whether the driver who crashed into the back of his Brighton and Hove Buses colleague suffered a medical episode.
One witness, Stuart Quinn, 41, told how he saw the bus travelling along with a large traffic cone wedged underneath it moments before the crash.
Mr Quinn, a shopkeeper, said: "It was making quite a racket. I thought to myself, 'How could he not notice that?'.
"As I looked closer, I looked at him. I looked away for a short time and then heard a God almighty crash and he had gone into the back of the stationary bus.
"It had been going at a rate of knots before the crash, definitely more than 20mph. He was conscious as we got to him but was breathing erratically. We shouted at him to try to bring him round."
Locals told how people from nearby shops poured out to offer help as the injured got off the buses. Fire crews, police, paramedics and an air ambulance were scrambled to the scene.
Two people, the driver of the rear bus and a male passenger, were taken in a life-threatening condition to Brighton's Royal Sussex County Hospital. Five more were also admitted in less serious condition.
Six more casualties went to the Princess Royal Hospital in Haywards Heath, West Sussex, and 10 were assessed at the scene but did not need to go to hospital, South East Coast Ambulance Service (Secamb) said.
At the scene, a road cone was seen wedged under the front end of the bus which had careered into the other double-decker. The rear bus windscreen had fallen out, while its upstairs windscreen was shattered.
Brighton and Hove Buses managing director Martin Harris said: "We are exploring all the possible causes and that includes the suggestion that there may have been some sort of medical episode involving the driver."
As a clear-up operation was under way, Inspector James Biggs, of Sussex Police, said it was too early to say whether a medical episode was to blame for the crash.
Speaking at the scene, he said: "As you can appreciate, on a sunny day in Brighton there were lots of people everywhere.
"It was very chaotic. The main priority was care for the patients. It was declared a major incident very early on.
"We got a very good response from the three emergency services, and we worked very well in the initial stages to make sure the patients got the care and treatment that they needed."
East Sussex Fire and Rescue Service said firefighters "released two casualties into the care of South East Coast Ambulance. Crews remained on the scene working alongside other emergency services.
"A total of seven casualties including two male casualties in a life-threatening condition have been taken to the Royal Sussex County Hospital in Brighton.
"A further six patients with non-life threatening injuries have been taken to Princess Royal Hospital in Haywards Heath and a number of other patients were checked over by Secamb clinicians for minor injuries before being discharged."