An investigation is under way after eight children fell from an inflatable slide at a fireworks display.
Woking Park in Surrey was evacuated and a major incident declared after the children - all under the age of 16 - fell at about 19:30 GMT on Saturday.
Seven were discharged from hospital overnight, with an eighth kept in for observation, with injuries "not believed to be significant".
The Health and Safety Executive (HSE) is investigating.
The site was evacuated at about 20:00 to allow the helicopter to land and "multiple crews" from the South East Coast Ambulance Service (Secamb) to help the injured.
Fireworks organiser Michael Holden, of Woking District Rotary, said it appeared the children had fallen to the ground from the top of the slide.
He said: "We don't still know yet what exactly happened but eight children appear to have come off near the top of the slide and landed on the floor alongside it."
He estimated about 5,000 people were in the park at the time.
One eyewitness told BBC News correspondent Simon Clemison there were "about 40 children" on the slide, which is about 30ft (9m) high when fully inflated.
Chris Tierney said he removed his three children from the slide "a couple of minutes before the incident" as he "felt the slide had become overcrowded".
He said his daughter was "getting bundled on by bigger children... kids were getting injured and crying. It was a disaster waiting to happen."
At the scene
By Tom Pugh, BBC News
Amid the merry-go-rounds, win-a-prize stalls and bumper cars at Woking Park lies the deflated inflatable at the centre of Saturday night's incident.
The park remained sealed off on Sunday morning as police maintained a low-key presence at the scene.
One witness, Andy Datson, 23, described seeing children being tended to by emergency crews.
Another described hearing an announcement over the public-address system which first alerted visitors to a "serious" incident.
Thousands of people are said to have been here on Saturday night to enjoy what should have been a fun night.
Later, scene-of-crime officers arrived at Woking Park to carry out inquiries.
All the children injured were aged under 16, Secamb said.
Those taken to London hospitals were treated at St George's Hospital and King's College Hospital, while one was treated at St Peter's Hospital in Chertsey, Surrey, a spokeswoman said.
The fireworks display was abandoned shortly after the children were injured.
Eyewitness Oliver Trimble told the BBC a message went over the event's loudspeakers announcing the evacuation shortly before 20:00.
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 had planned to take her daughters, aged 10 and six, on the giant slide after the fireworks.
"We saw all the blue lights and saw the air ambulance arriving. We all evacuated very calmly. Everyone was just shocked," the 36-year-old said.
Mr Holden said: "We have used this funfair operator for a number of years. We have never had any problems before. [The operator] is as upset as we are.
"We are very shocked and distressed by the whole thing, but we are relieved to hear this morning that seven out of the eight children have been discharged from hospital."
The park will remain closed on Sunday while police investigate the cause of the incident, the club added.
John Edwards, of Edwards Amusements, who hired the slide on behalf of the rotary club, described it as an "unfortunate accident."
He told the BBC: "I do not know what happened", but said the slide did not collapse, the children fell.
He said the slide had been removed by the HSE and he was now dismantling the rest of the fairground.
The funfair was put under police cordon and investigators were inspecting other attractions, including the crooked house and the Ferris wheel.
A Woking Borough Council spokesman said: "We will be working with the police and relevant authorities to understand and explain what happened."
Officers have appealed for any eyewitnesses or those with video or photos of what happened to contact them.
Mark John, from Airquee, an organisation that inspects inflatables, said the items should go through rigorous checks to ensure they are safe.
"Inspections can take anything from a minimum of 45 minutes right up to several hours," he said.
"As people are aware lately there's a lot of emphasis on making sure the anchor points are in place and intact as they should be for protection against gusts of wind or strong winds."
an inflatable slide at a funfair near me just collapsed and loads of kids are seriously injured !!! my heart goes out to everyone affected and I’m praying for them to recover soon❤️— rara (@aestheticdolxnx) November 3, 2018
Joe Mercer, general secretary of the Showmen's Guild, told the BBC: "This slide has up-to-date test certificates and insurance documents, these have been provided to police. Our members are fully co-operating with the authorities; no arrests have been made."
He added: "Members of the Showmen's Guild operate equipment at fairs to strict safety standards that are subject to HSE scrutiny. Our thoughts are with those affected by this incident."
A spokeswoman for the HSE said: "We are aware and making initial enquiries."