The collapse of an inflatable slide at a fireworks funfair that resulted in eight children being taken to hospital is being investigated.
Seven of the eight children, all aged under 16, have been discharged following the incident in Woking, Surrey.
One boy was kept in for overnight observations but suffered injuries “not thought to be significant”, police said.
One witness said he was concerned earlier that evening when he saw up to 40 children playing on the “flimsy” slide at once.
Chief inspector Steve Schulten said: “A number of children fell from the slide and suffered injuries.
“Eight children have been taken to hospital by South East Coast Ambulance Service with potentially serious injuries.”
He added the Health and Safety Executive has been informed of the incident.
Secamb had “multiple crews” at the scene of the “significant incident”.
The patients were being transported to “major trauma centres” in London with the help of the air ambulance, a statement added.
Andy Datson, 23, said he saw up to 40 children playing on slide, which he estimated was about 30-foot tall at its highest.
“We had been walking past the slide earlier in the night and said it looked unsafe. It looked pretty flimsy to say the least,” he said.
“There were far too many kids on it. It didn’t look like it could hold that many people.”
The journalist, who lives in Guildford, Surrey, said that he noticed the flashing lights of ambulances after walking off with friends.
“I immediately thought that’s going to be the slide,” he said.
“It looked like there were too many children on it and the side area had given way.
“If, heaven forbid, they fell from the top I would say it’s at least 25 to 30 feet tall.”
Oliver Trimble told the BBC that the evacuation was announced over the event’s loudspeakers shortly before 8pm, when the fireworks were due to begin.
He said he saw victims being treated by medics, adding: “It was quite a horrendous sight to see.”
“This is one of Woking’s biggest events of the year, thousands of people come to see the fireworks,” he said.
Local musician Anna Neale said she was among the hundreds of people who evacuated the park, which had been packed with families with young children.
The 36-year-old said: “We saw all the blue lights and saw the air ambulance arriving. We all evacuated very calmly. Everyone was just shocked.”
She had planned to take her daughters, aged 10 and six, on the giant slide after the fireworks.
She said: “I am very grateful my children didn’t go on it, let’s just put it that way.”
Woking District Rotary Club, the event’s organisers, said it was “shocked and distressed” by the collapse.
The park will remain closed into Sunday as police investigate the cause of the incident, the club added.
A Health and Safety Executive spokeswoman said “initial enquiries” were being made.
A Woking Borough Council spokesman said: “We will be working with the police and relevant authorities to understand and explain what happened.”